President Recep Tayyip Erdogan entered this year facing a knot of political problems with little precedent in his two decades at the summit of power in Turkey.

Voters were angry about persistently high inflation. His political party’s popularity had sunk. And his opponents had coalesced around the mayor of Istanbul, Ekrem Imamoglu, who made it clear that he was gunning for the presidency.

Then on Wednesday, just four days before the mayor was set to be designated as the political opposition’s presidential candidate, dozens of policemen arrested him at his home on accusations of corruption and terrorism.

Mr. Erdogan’s foes consider the arrest a ploy to abort Mr. Imamoglu’s presidential campaign before it even begins. At stake is not only who will be Turkey’s next president, analysts, opposition leaders and foreign officials say, but to what extent Turkey, one of the world’s 20 largest economies and a U.S. ally in NATO, can still be considered a democracy.

“Turkey has never been a perfect democracy, but arresting a presidential candidate is taking this imperfectness to another level,” said Arife Kose, a doctoral candidate who studies Turkish politics at the University of East Anglia in Britain. Using the state’s power to foreclose competitive elections, she said, “means that it is getting closer to a fully authoritarian country.”

Mr. Erdogan has dominated Turkish politics since 2003, first as prime minister then as president since 2014. During that time, he has overseen tremendous economic growth and repeatedly led his ruling Justice and Development Party to victory at the polls.

But over the last decade, his critics say, he has solidified his control by eroding Turkish democracy, stocking the state bureaucracy with loyalists, co-opting the news media to limit negative coverage and cultivating state prosecutors and judges to legally punish his foes.

Still, most experts have not considered Turkey an outright autocracy, because many civil freedoms remain and opposition parties have contested elections — and sometimes won, as they did in municipal races across the country last year.

The question now, analysts said, is whether Turkey will remain a mix of democracy and autocracy or shift significantly toward the latter.

Barring Mr. Imamoglu from the presidential race would put Turkey in league with countries like Russia, Belarus or Azerbaijan, where elections happen but make little difference, said Hasan Sinar, a professor of criminal law at Altinbas University in Istanbul, who is also defending another elected Istanbul official accused of terrorism links.

“They have elections, but they are so-called elections because the president himself is designing the opposition and deciding who is going to run against him,” he said.

Mr. Erdogan on Thursday dismissed opposition calls for protests over the mayor’s arrest as “theatrics.”

“The opposition never responds to the allegations brought forward by the judiciary,” he said. “Instead, they confine the matter to political slogans, resorting to the easy way of provoking their base and deceiving the public.”

The political turmoil came as Turkey found itself well positioned to benefit from recent global events. A rebel group it supported leads the new government in Syria. The Trump administration has shown little interest in whether its foreign partners follow democratic standards. And concerns that the United States will stop supporting Ukraine in its war with Russia have pushed European leaders to seek stronger defense ties with Turkey.

Those interests could blunt foreign criticism of Mr. Erdogan’s governance, analysts said. U.S. officials have said little about Mr. Imamoglu’s arrest, but some European leaders have expressed concern.

Chancellor Olaf Scholz of Germany on Thursday called the arrest a “very, very bad sign” for Turkey’s relations with the European Union.

“We can only call for this to end immediately and for the opposition and the government to stand in competition with each other, and not the opposition being brought to court,” he said.

Mr. Erdogan’s current, second presidential term ends in 2028. The Constitution allows only two terms, but he could legally run again if Parliament called early elections, which are widely expected. That could put Mr. Erdogan, 71, on the ballot against Mr. Imamoglu, 54.

The mayor’s arrest followed a series of recent government moves against perceived critics.

Since January, a well-known journalist and an agent who represents famous actors have been arrested in connection with anti-government protests more than a decade ago that the state suppressed and criminalized.

Last month, two top officials in a prominent business association criticized the government’s economic program, the prosecutions of prominent figures and insufficient adherence to the rule of law. State prosecutors indicted them both on charges of spreading false information and recommended prison sentences of up to five and a half years.

Since October, the authorities have removed three Istanbul district mayors on accusations of corruption and terrorism. One has been replaced with a government appointee.

Less prominent figures have also fallen afoul of the government. An astrologer was detained last month and accused of insulting Mr. Erdogan and another senior politician. The Trade Ministry investigated a food vlogger who positively reviewed a restaurant subsidized by Mr. Imamoglu’s city government to see if he had been paid to do so.

Mr. Imamoglu became mayor in an upset victory in 2019. The government got the results thrown out, citing alleged irregularities, but in a redo, Mr. Imamoglu won again by an even larger margin. He was re-elected last year, defeating a candidate backed by Mr. Erdogan.

During his time as mayor, the government has launched 42 administrative and 51 judicial investigations of Mr. Imamoglu, his aides said, which together seek to hobble his administration and remove him from the presidential race. In one case, he stands accused of corruption during a previous job as a district mayor. He was convicted in another of insulting public officials by calling the judges who overturned his initial victory in 2019 “fools.” He has appealed the verdict.

Before his arrest this week, his alma mater, Istanbul University, announced that it had annulled his diploma, citing improper procedures in his transfer from a university in Turkish-controlled Northern Cyprus in 1990. He has vowed to appeal, but if the decision stands, it could bar him from the presidency because the Constitution stipulates that the president must have a university degree.

Despite these roadblocks, Mr. Imamoglu’s popularity has remained high, making him a threat to Mr. Erdogan, said Berk Esen, an associate professor of political science at Sabanci University in Istanbul.

“He made it clear to Erdogan that the train is moving, it is coming in his direction, and he won’t be able to stop it with the normal means,” Mr. Esen said. As a result, Mr. Erdogan “went for the jugular.”

Prosecutors have accused Mr. Imamoglu of leading a criminal organization and overseeing bribery, bid-rigging and other crimes at City Hall. He stands accused in a second investigation of supporting terrorism through his political coordination with a pro-Kurdish group.

Turkey’s opposition has vowed to go ahead with the primary on Sunday to name him its presidential candidate and called for protests against his detention.

The government has banned public demonstrations in Istanbul, Ankara and Izmir, shut down main streets and subway stations, and restricted access to social media platforms that activists use to organize. It has called on people to trust the legal process and insisted that the courts are independent.

“Attempting to associate judicial investigations and cases with our president is, to say the least, an act of audacity and irresponsibility,” Justice Minister Yilmaz Tunc told reporters on Wednesday, calling the separation of powers “a fundamental principle.”

“The judiciary does not take orders from anyone,” he said.

Every day, thousands of protesters gather near South Korea’s top court, which has fortified its walls with razor wire as its eight justices prepare a ruling that could shape the future of the country’s democracy. Many shout for the court to remove Yoon Suk Yeol, the country’s president, accusing him of “insurrection.” Nearby, a rival group chants for his reinstatement, calling his impeachment by Parliament “fraudulent.”

Never have the jitters run so high in South Korea before a court ruling as they do now, while the country waits impatiently for its Constitutional Court to decide whether to remove or reinstate Mr. Yoon. The court’s ruling could help bring an end to the months of political turmoil Mr. Yoon unleashed on Dec. 3 with his failed attempt to place his country under martial law.

Or it could push the country into a deeper political crisis.

South Koreans have grown weary of the prolonged political uncertainty and want the court to decide soon. But it has kept them on edge for weeks, giving no hint when its justices will deliver probably the most consequential ruling of their careers.

Police are preparing for the worst, regardless of how the decision goes. Schools, street vendors and a former royal palace in the neighborhood of the Constitutional Court will close on the day of ruling. The police called for the removal of rocks, empty bottles, trash bins — anything that could be a weapon — from the streets. About 14,000 police officers are ready to deploy. Drones have been banned from flying over the courthouse, and all 86,000 privately owned guns in the country must be kept locked in police stations.

As the deliberations dragged on, conspiracy theories abounded. Some speculated that the court was irreparably split. (The votes of at least six of the eight justices are needed to remove Mr. Yoon. Otherwise, he will be reinstated.) Others said it was just taking time to make its historic ruling watertight.

“The country is at a crossroads,” said Cho Gab-je, a prominent South Korean journalist and publisher who has covered the nation’s political evolution since 1971. “If the court reinstalls Yoon in office despite what he did, it will give future leaders of the country a license to rule by martial law. South Korea will become a banana republic.”

Mr. Yoon’s martial law lasted only six hours but provoked fury among many South Koreans. The National Assembly impeached him on Dec. 14, suspending him from office. If Mr. Yoon is removed, South Korea will elect a new president within 60 days.

Polls in recent weeks showed that South Koreans preferred Mr. Yoon’s ouster to his reinstatement roughly 3 to 2. But in a deeply divided nation, Parliament’s decision to impeach him has also galvanized the political right, fueling their fear and hatred of their progressive political opposition.

No matter what it decides, the court will anger a large portion of the society. People on both sides have shaved their heads or gone on hunger strike to press their demands.

“If it endorses Yoon’s impeachment, there will be some commotion, but the political landscape will quickly shift toward a presidential election,” said Shang E. Ha, a professor of political science at Sogang University in Seoul. “But if he returns to office, even those who have been waiting patiently for a ruling will take to the streets. We will see riots.”

In a typical anti-Yoon rally, participants hold signs that call him “the ringleader of insurrection” and chant for his “immediate ouster.”

But far-right Christian pastors and lawmakers who supported Mr. Yoon threatened to “shatter” the court should it decide to remove him. In January, scores of Yoon supporters vandalized a Seoul district courthouse after a judge there issued a warrant to arrest him on an insurrection charge. Two supporters of Mr. Yoon died after setting themselves on fire in protest of his impeachment.

The police also assigned bodyguards to Lee Jae-myung, the main opposition leader, after his party reported anonymous assassination threats. Officers have escorted the eight justices on their commute to and from the court.

“I urge the people to respect and accept whatever the court decides,” said Finance Minister Choi Sang-mok, the country’s acting president, while expressing concern over the potential for violent clashes.

When the court ruled to remove another impeached conservative leader, President Park Geun-hye, in 2017, there were no violent deaths in the resulting protests, though four people rallying in her support died, either from heart failure or being hit by a falling object.

In an increasingly polarized era, Mr. Yoon’s impeachment trial is even more emotionally charged.

In 2017, rival political parties agreed weeks before the court’s ruling that they would honor it. Ms. Park waited in silence for the ruling, while her party distanced itself from the leader disgraced for corruption and abuse of power.

Mr. Yoon, who also faces a criminal charge of insurrection, has showed no intention of going quietly. His People Power Party and the country’s mostly older and churchgoing right-wing voters have rallied behind him.

He has defended his declaration of martial law as an effort to save his country from the “dictatorship” of the opposition-controlled National Assembly and “anti-state forces.”

But the political instability Mr. Yoon set off has left his country without an elected leader at its helm at a time when North Korea is escalating its nuclear threat and strengthening military ties with Russia. Although South Korea is one of America’s key allies, its leader has yet to meet President Donald J. Trump while leaders of other nations, including Japan, have.

U.S. Defense Secretary Pete Hegseth has decided to skip South Korea in his upcoming trip to the Indo-Pacific region while the political limbo remains. In February, the Economist Intelligence Unit’s Democracy Index 2024 downgraded South Korea from a “full” to “flawed democracy.”

In his last argument at the Constitutional Court last month, Mr. Yoon said that if allowed to resume the presidency, he would leave domestic affairs to the prime minister and focus on diplomacy. But Mr. Lee, the opposition leader, said Mr. Yoon has already done enough damage to the country’s global image.

“We can overcome the current crisis only when we restore normal leadership,” he said.

Heathrow Airport in London, one of the world’s busiest air hubs, was set to resume a full day of service on Saturday. But after a fire at an electrical substation halted operations for most of Friday, global travel remained disrupted.

The police were still investigating what had caused the fire at the substation in western London that cut power to tens of thousands of nearby homes as well as to Heathrow. The Metropolitan Police said there was no immediate indication of foul play.

British Airways, Heathrow’s largest carrier, said late Friday that it expected about 85 percent of its nearly 600 departures and arrivals scheduled for Saturday to go ahead but that delays were likely to affect all passengers. The airline said it was also canceling flights on high-frequency routes where passengers have more rebooking options.

It took 16 hours to end the disruption that began in the early morning hours of Friday. More than a thousand flights were diverted, wreaking havoc on more than a quarter of a million people’s travel plans, Cirium, an aviation data company, estimated.

Arrivals resumed Friday evening. A Heathrow representative said significant delays were expected in the coming days as airlines tried to return their planes to their usual schedules.

“Please do not travel to the airport unless your airline has advised you to do so,” the airport said on Friday. “We hope to run a full operation tomorrow and will provide further information shortly.”

Planes from all over the world were heading to Heathrow early Saturday, including from Hong Kong, South Africa and Brazil, according to Flightradar24, a tracking website.

Thomas Woldbye, Heathrow’s chief executive, said that at the time of the substation fire, a backup transformer was working as it should when the airport lost power but that it was not enough to run the entire facility. The London Fire Brigade said that it had been difficult to extinguish the blaze because the substation held thousands of gallons of cooling oil.

Britain’s National Grid said that the substation’s network was reconfigured to partly restore power temporarily to the airport and other customers.

Some travelers were headed to weddings, some to funerals. Others were leaving for a Disneyland Paris vacation, taking a business trip to Britain, returning home from a honeymoon in Italy or moving to Australia.

Instead, on Friday, they found themselves sitting on airport floors, riding on buses or checking in to hotels as tens of thousands of passengers were stranded — unable to reach their destinations because of a power outage that shut down Heathrow Airport.

“It’s a crazy situation,” said Roxanna Bagherzadeh, who was missing Persian New Year’s celebrations with her family in London because her flight from New York never took off.

Some flights were rerouted midocean for Madrid or Montreal. Others were forced to turn back entirely. American school trippers were being held in London. Musicians worried they would miss their performances. Some travelers took circuitous detours across Europe to reach their families. Others had to give up on big plans.

“I was going to propose,” said Kevin Black, a bass player touring Europe with his band who was supposed to be joined by his girlfriend. But her flight turned around and took her back to Nashville.

“It was going to be on one of the bridges overlooking the river at sunset” in Paris, Mr. Black said of his planned proposal on their European trip. Now, he said, “it will probably be at a waterfall in Tennessee. What can I do?”

With airlines struggling to respond to huge amounts of requests, many passengers could not get through to them. Flights for the next few days quickly booked up, and the only ones left had exorbitant prices, travelers said.

The outage also came as many Americans were starting their spring break, disrupting family plans.

“Instead of fish and chips we have crying children at home,” said David Mahler, 47, who was flying with his wife and 11-year-old twin boys to London from Los Angeles when their flight was turned around over Colorado. They were now back home looking for other options for spring break, he said, but international flights had become too expensive. “We are kind of stuck,” he said.

While passengers at London’s second-largest airport, Gatwick, which received many flights that had been headed to Heathrow, stood in long lines at help desks. Heathrow, which is usually one of the world’s busiest airports, was deserted. The drop-off zone at Terminal 3 was silent, and runways and airline counters were empty.

A teacher who had been trying to get home to Dallas tried walking between terminals when the bus she was on was not allowed to enter Heathrow. Another traveler, Monel Bailey, tried to walk to the terminal along the highway with his bags when his Uber was stopped by police officers blocking the route to the terminal. It was “a scene of chaos,” Mr. Bailey said.

The power outage, caused by a fire at an electrical substation near Heathrow, forced the authorities to close the airport for most of Friday. While some flights were set to resume later Friday, airline officials warned that travel disruptions could last for days.

A glance at Heathrow’s flight information board on Friday morning gave a sense of how big the shock waves from the closure would be. Flights from Brunei, India and Vietnam were scheduled to land, and passengers had been expecting to board planes to dozens of destinations: Miami, Singapore, Tokyo … the list went on.

Rachel Morris was eagerly waiting for her sister to join her in Philadelphia for her bridal shower on Saturday, but her flight from London was canceled. “I am devastated,” Ms. Morris said. “She is my maid of honor and my best friend and she planned the entire thing.”

Stephen McCray was stranded in London, where he had traveled from Seattle for a book tour for his wife’s debut horror novel.

“We want to get home,” Mr. McCray, 36, said. “Our dogs are waiting for us.”

Travelers took to social media to complain about the disruption and beseech the airlines for help. One wrote that he had had to pull out of a Pokémon Go championship, another that she could no longer run in a half-marathon for which she had trained.

“Sad to miss it,” said the half-marathon runner, Samira de Blij, who had been planning to fly from Amsterdam for the race in Reading, west of London, on Sunday.

Ms. Bagherzadeh posted on TikTok what she called “the saddest travel vlog of all time.” In it, she sat on the plane, took the first step of her in-flight skin care routine and braided her hair so it wouldn’t get messy on the flight.

“It was all for nothing,” she said on a phone call.

Ms. Bagherzadeh, 27, had intended to have dinner with her grandmother, who had traveled to Britain from France for the occasion. Instead, she sat on a plane on the tarmac for two hours before disembarking and returning home.

“I had a quick cry, because I wanted to see my mom,” Ms. Bagherzadeh said.

Dozens of airlines fly to Heathrow from about 180 cities worldwide. Many planes already in the air were forced to divert elsewhere when the airport closed. John F. Kennedy International Airport in New York had the most flights scheduled to arrive at Heathrow on Friday; some were diverted to Manchester, in northwestern England; to Glasgow; or to Reykjavik, Iceland.

As many as 290,000 passengers traveling in or out of Heathrow could be affected by the closure, according to Cirium, an aviation analytics company. On Friday, 669 flights were scheduled to take off from Heathrow, Cirium said.

Heathrow is the main hub for British Airways, which said that it was redirecting flights to other airports in Britain when possible.

At Fiumicino Airport in Rome, dozens stood in line at the British Airways counter, including a group of high school students from Arizona who had planned to fly home via Heathrow after a week in Italy.

Among the chaos, panic and frustration, some still tried to look at the upside.

Marilyn Leblanc said she was flying from Boston to Dublin when the pilot announced that because of the fire at Heathrow they would be heading back. But because the plane needed refueling, they were diverted to Canada for fuel.

“So we left Boston at around 7 p.m. Thursday night and arrived back Friday morning at 5 a.m.,” Ms. Leblanc said. “Just a nice Sunday drive around the ocean!”

Reporting was contributed by Nicholas Yong from Singapore; Jonathan Wolfe and Lynsey Chutel from London; and Matthew Mpoke Bigg from Rome.

Irma González recognized the gray backpack in the photo. It was the same one her son had used for high school and the one he had taken with him for his first job three years ago, just before he vanished.

When Ms. González, 43, saw the images on television of bone fragments and scattered personal belongings uncovered on a ranch in western Mexico, her heart sank. Had her son, Jossel Sánchez, met his fate there? Were his remains there somewhere? Or had a criminal group brought him to that place only to take him elsewhere?

Standing about 300 feet away from the entrance of the Izaguirre ranch on Wednesday, surrounded by sugar cane fields and barren hills, she was desperate for answers.

“I just want to find my son, dead or alive,” she said while sobbing and pleading with local police officers who had cordoned off the site to let her inside.

Ms. González echoed the sorrow felt by countless other Mexicans searching for missing loved ones, which has been shattered by a mixture of hope and despair. This emotional turmoil followed the discovery by search volunteers two weeks ago of a ranch outside La Estanzuela, a small, dusty Mexican village near Guadalajara in Jalisco state.

Inside the abandoned site, members of the search group, called Searching Warriors of Jalisco, found traces of unimaginable violence: cremation ovens, burned human remains and bone shards. Discarded personal items, and hundreds of shoes.

The discovery has sent shock waves through the nation, becoming the latest symbol of Mexico’s relentless violence and its crisis of disappearances.

More than 120,000 people have gone missing in Mexico since the country started keeping track in 1962, according to official data. From 2018 to January 2023, the government agency that coordinates efforts to locate missing persons in Mexico recorded 2,710 clandestine graves containing human remains across the country.

So far local authorities do not have many answers about the so-called “extermination camp” here in Jalisco, as media outlets and the search group have come to call it. Officials have said the camp may have been operated by the Jalisco New Generation Cartel — one of the most violent criminal organizations in the country — to train recruits, torture their victims and dispose of bodies. But they have yet to say how many people died on the site, and none of the remains have been identified.

On Wednesday, Attorney General Alejandro Gertz of Mexico criticized the initial investigation carried out by local authorities and said it had been riddled with irregularities. Local officials failed to secure the site after it was first located six months ago by National Guard members, and it was “abandoned” soon after, Mr. Gertz said.

Those investigators did not properly document or register evidence that they found on the site, nor did they take fingerprints found in the place, he said. The country’s attorney general’s office has since taken over the investigation at the request of President Claudia Sheinbaum.

New York Times journalists went inside the football field-sized camp enclosed by cement walls on Thursday.

All the evidence uncovered by the search group was gone — collected by authorities and dozens of investigators, law enforcement officers and forensic experts. Small yellow flags punctuated the desolate terrain, each one marking a spot where investigators had uncovered a piece of evidence.

Inside a large warehouse with a tin roof, where the search group discovered piles of clothes and shoes, the space now stood eerily empty. Three chickens wandered through the silence. On the floor, a single candle flickered.

Trash, empty beer cans and shards of broken glass littered the ground. Partially buried car tires and barbed wire marked the area where authorities believe the cartel may have trained its recruits.

Small holes, no larger than a trash bin, dotted the earth like a saltshaker, left by forensic anthropologists who excavated the soil in search of human remains or other evidence.

Several larger dig sites were cordoned off by yellow police tape.

The day prior, Ms. González had eventually been allowed in, only to discover all the evidence had been relocated. She left there with a mix of relief and disappointment. “As a mother I am relieved, but I want to end this suffering,” she said.

Over three years ago, Ms. González’s son Jossel disappeared after being recruited for a cellphone store job in Puebla, in central Mexico, through a Facebook ad. At 18 and nearing graduation, he dropped out to support his family when Ms. González fell ill with pneumonia that left her unable to work.

Soon after news of the extermination camp emerged two weeks ago, authorities published a catalog with photographs of more than 1,500 items found inside the ranch. Ms. González said she had recognized Jossel’s backpack.

She gathered enough money to buy a plane ticket to Jalisco to see for herself if the backpack truly belonged to her son. Perhaps, in that small confirmation, she could find some clarity, and maybe even some peace.

Numerous families from across Mexico have scoured the photos, desperately searching for signs of their missing relatives. Some have recognized items and rushed to Guadalajara, Jalisco’s capital, hoping to find answers.

While the discovery of the ranch shocked the nation, news of the emergence of new mass graves and buried victims has become a common occurrence in Jalisco state, which has the highest number of disappearances in Mexico.

Only two days before the Izaguirre ranch was found, members of the Searching Warriors of Jalisco group got a tip about a mass grave on a residential property in Guadalajara. There, they uncovered 13 bags containing human remains buried in the backyard, according to Raúl Servín, one of the leaders of the search group.

The residents were unaware of the grave’s existence, he said.

Seven years ago, Mr. Servín was forced to become an anthropologist of sorts when his 20-year-old son, Raúl, vanished without a trace. It was a woman from a different search organization who taught him the skills he would need: how to choose the right shovel for digging and to recognize the specific hollow sound the earth makes when stepped on — a telltale sign that something, or someone, might be buried beneath.

He now splits his days working as a waiter and responding to hundreds of calls with tips of possible locations of mass graves across Guadalajara. He goes, shovel in hand, inspects the terrain and digs, looking for missing victims. In seven years, he said, he has found hundreds of bodies.

He does it to try to give the families some peace.

“A pair of shoes does not give you a body to bury and go visit in a cemetery, or any clarity of what happened to my boy,” said Mr. Servín, 53.

His son is among the more than 15,000 people who have gone missing in the state of Jalisco. Many of these cases are believed to be linked to the Jalisco New Generation Cartel.

As the criminal group has expanded its territory across the state in recent years, the number of homicides and disappearances in Jalisco have rocketed.

Ulises Ruiz, a local photographer who was with the search group when they discovered the ranch site earlier this month, likened the widespread disappearances in Jalisco to a pandemic, noting the phenomenon has grown exponentially, affecting more and more people.

“Like it happened with Covid, we thought it was happening somewhere else, in other states or cities,” he said. “But suddenly, everyone around you has a loved one or knows someone who has vanished.”

James Wagner contributed reporting.

Phillip Kizun found himself having to improvise on Friday as he tried to get from London to Dublin, typically a routine trip.

After finding out that his flight from Heathrow Airport had been canceled, Mr. Kizun, 58, took a train to Wales and then a ferry from the coastal town of Holyhead to the Irish capital. He met several European and American travelers who were doing the same.

“It was an absolute real ‘Planes, Trains and Automobiles,’” Mr. Kizun said, minutes after arriving in Dublin for a work trip, referring to the 1987 Steve Martin-John Candy comedy.

Mr. Kizun, 58, of Chester County, Pa., was among the thousands of anguished travelers who found themselves stranded on Friday, after a fire at an energy substation near Heathrow caused a power outage that closed the airport for the better part of a day. Some Heathrow-bound flights turned around in midair, while many others did not take off, leaving confused and anxious passengers wondering when they might finally get where they were going.

The New York Times asked readers to share their stories. Some, like Mr. Kizun, found alternative routes. Others remained in a state of limbo. A few simply gave up.

Planes were turned around midflight.

Some planes that were already in the air turned around. That was the case for the one carrying Jeannie LaChance, who was traveling to London from Los Angeles with her sister and 2-year-old niece. About four hours into the flight, the pilot let the passengers know that there was a possible fire at Heathrow and that they would have to return.

“Everyone was pretty calm, which I think was nice because we’re all trapped in a plane,” Ms. LaChance, 31, said.

Henry Kofman, 20, a sophomore at the University of Southern California, chose London as a spring break because of its theater scene. He was having such a good time that he decided to extend his trip by a day.

Tough luck. He was up late watching the “Severance” season finale when he got a news alert on his phone that Heathrow had been shut down. It seemed unreal.

“I just don’t find it believable that the busiest airport in Europe is just gone for the day,” he said.

Mr. Kofman is hoping that operations return to normal on Saturday, as airport officials have promised, so he can be back in Los Angeles in time for classes to resume on Monday.

Some travelers changed their itineraries.

Cyndi Darlington, a marketing executive, was set to take her best friend on her first overseas trip, to London and Rome. The pair sat on the plane for three hours at the San Diego airport. When they were told that the flight had been canceled, Ms. Darlington and her friend moved swiftly to reorganize their trip. Now they will head straight to Rome, cutting two days off their vacation.

“We overheard people on the plane talking about going to a wedding and making connections,” Ms. Darlington said. “And so we feel that we’re kind of lucky. And we only lost two days.”

The outage forced some passengers to cancel their trips entirely. Iris Planamento was on a plane at Newark Liberty International Airport on Thursday night, waiting to take off for a package tour that would take her to London, Normandy and Paris. After her flight was canceled, she visited an online chat forum for the tour, where people were sharing stories about being turned back midair or otherwise being diverted or delayed. Not wanting to deal with the stress, Ms. Planamento, 72, canceled her trip.

“I have to tell you that I am not a person who has anxiety problems,” she said. “For the past few weeks, I’ve been feeling a lot of anxiety. I’ve never felt that before, and yesterday was especially bad until I got to the airport and had a few drinks. Today I’m fine. I feel I’m disappointed. But the anxiety is gone.”

Camille Dee, 74, of Roslyn, N.Y., and her husband were supposed to take off from Kennedy International Airport for London on Friday night for what she described as their first vacation in six years. Their flight was canceled, and the earliest rescheduled flight that the airline is offering her is on Monday. Worried that the new itinerary may cost her hundreds of dollars in nonrefundable hotel fees and other associated costs, she is considering canceling the trip, though she did allow that her travails amounted to a “first-world problem.”

“After this, it’s going to be a while before I decide to go back to London, because this whole thing has left such a bad taste in my mouth,” Ms. Dee said.

‘A little unnerving’: The closure fueled anxiety.

Alyse Franklin, 22, a senior at Indiana University, was stressed out, not knowing when she would be able to make it home from a spring break trip with 19 classmates from her international marketing and communications class.

“Flying in general kind of makes me feel a little ill,” Ms. Franklin said. “So it’s not fun to fly already, but the fact that we don’t know when it’s going to happen and it feels like I can’t mentally prepare for it, it’s a little unnerving.”