He blocked Facebook, Instagram and Twitter.

He signed a censorship law that led TikTok to disable its functions.

President Vladimir V. Putin has clamped down on free expression in Russia to a degree unseen since the Soviet era. Now he is taking aim at the last Western tech platform barely standing in wartime Russia: YouTube.

Mr. Putin has not formally banned the U.S. video platform that has more than 2.5 billion users worldwide. But the site has angered Russian authorities, who view the site as an uncontrollable gateway to antiwar content. They have also decried YouTube for removing Russian propaganda channels as well as videos by Russian musicians subject to western sanctions.

So last summer Russian users experienced a significant slowing of YouTube, primarily on desktop internet connections. Internet experts have said the sudden and simultaneous drop-offs in traffic could be explained only by deliberate throttling of the service on the part of Russian authorities.

The purposeful slowing of the service spread to a wider swath of the internet, including mobile networks, last month. Millions of Russians trying to access videos have found them too slow to load or too pixelated to watch.

“This sudden massive drop is 100 percent artificial,” Philipp Dietrich, an analyst at the German Council on Foreign Relations, said. “There is no doubt about the fact that this is human-made.”

The results of the broadside against YouTube have so far been mixed, demonstrating the complications Moscow faces in snuffing out an American-made cornerstone of the Russian internet that for years was seen as practically too big to ban.

YouTube for years has been a staple of daily life for many Russians, streaming everything from old Soviet movies to anti-Kremlin political shows. Some 96 million Russians over the age of 12, or about 79 percent of the over-12 population, visited the site monthly as of July, before the slowdown in service began, according to the research group MediaScope.

But the relationship between the Kremlin and Google, which owns YouTube, has been tense for years. Searing viral YouTube broadcasts transformed the late Russian opposition figure Aleksei A. Navalny into a significant threat to the Kremlin. His corruption investigation into a palace on the Black Sea built for Mr. Putin, released on YouTube in early 2021, has drawn 133 million views over the past four years, underscoring the power of the platform.

On one level, the throttling looks to have worked. Russian internet traffic to YouTube is less than a third of what it was this time last year, according to public data released by Google, the streaming service’s parent company. VK, the state-controlled social media network, is pitching a domestic alternative to YouTube, known as VK Video, and it has trumpeted surges in traffic.

But the reality is more complex.

Droves of tech-savvy Russians are continuing to access YouTube using virtual private networks, or VPNs. Those tools route their internet traffic through another country, meaning it does not show up in Google’s data as Russian usage. They also encrypt users’ traffic and protect their identities.

The impeding of YouTube has also proved spotty across Russia’s hundreds of internet providers, leaving some Russians able to access YouTube videos directly, even without VPNs.

Political shows critical of the Kremlin filmed outside Russia have seen relatively minimal traffic declines from the slowing service, according to the Russian journalist Dmitry Kolezev, who tracks the shows through a product called YouScore. That is likely because their viewers in Russia who are particularly motivated to view anti-Kremlin content have swiftly acquired VPNs.

Entertainment content, ranging from children’s cartoons to cooking shows, has seen a significant drop-off in many cases, according to YouTube traffic measurement sites. Viewers of such content are less likely to purchase VPNs and may be able to find what they are looking for on Russian streaming platforms.

The exact number of Russians using VPNs is unclear. Mikhail Klimarev, executive director of the Internet Protection Society, a digital rights group now based in Europe, estimated that more than half of Russian internet users, or about 60 million people, at least know what a VPN is and say they are able to use one.

“People will learn to use VPNs because of YouTube and will discover that there is much more to the internet than what they get on the regular Russian internet,” Mr. Klimarev predicted. “It is simply of higher quality, there are simply more opportunities, more access to content.”

Still, the slowdown in service is driving many Russians to state-controlled domestic platforms, such as VK and RuTube, to consume at least some of the content they used to watch on YouTube. That is a bifurcation of the internet that the Kremlin desires.

“We are calling this phenomenon a splinternet,” said Anastasiya Zhyrmont, policy manager for Eastern Europe and Central Asia at the digital rights group Access Now. They are trying “to splinter the internet and build their own ecosystem,” she said.

Ilya Shepelin, a Russian journalist in exile who makes popular YouTube videos skewering state propaganda, worries that only politically oriented Russians willing to go through the process of setting up and paying for quality VPNs will end up staying on YouTube, with the rest migrating toward a state-controlled domestic internet for leisure, where they will not chance upon political videos critical of the state.

The result, he said, would be “a kind of information bubble” where video creators will not “reach the average Russian.”

Already, some bifurcation is visible.

Artur Dneprovsky, the creator behind some 20 YouTube channels showing Russian-language children’s cartoons, including the popular “Blue Tractor,” said in an email that his studio’s bigger channels have seen drops in YouTube traffic from 20 percent to 30 percent, while the smaller projects have dropped up to 50 percent, amid the slowdown.

At the same time, he said, he has seen noticeable and rapid increase in views and subscribers on Russia’s domestic video platforms, especially RuTube, where more than 400,000 people have signed up for “Blue Tractor” since the start of the throttling — suggesting that some people having trouble with YouTube are migrating to RuTube or VK as alternatives.

Maxim Katz, a Russian opposition figure who broadcasts a popular political YouTube show from Israel, watched as the number of users tuning into his show from Russia in the data for his channel dropped 45 percent from a year ago. But his overall viewership numbers stayed the same, suggesting that some viewers in Russia had adopted VPNs and were showing up in the data as coming from other countries.

“People simply switched to using VPNs en masse and are continuing to watch YouTube,” said Mr. Katz, who is on Russia’s federal wanted list and does not publish videos on the state-controlled platforms.

Mr. Putin’s invasion of Ukraine in early 2022 significantly escalated the Kremlin’s clash with Google. The company globally blocked more than 1,000 Russian state-sponsored propaganda channels, including more than 5.5 million videos, according to YouTube. It suspended ads shown on YouTube to users in Russia, as well as the serving of ads by Russia-based advertisers to users globally.

Google regularly denied demands by the Russian authorities to remove content. For example, after Mr. Putin announced a mobilization in September 2022 to shore up his reeling forces in Ukraine, Russia’s communications regulator asked Google to remove 63 videos from YouTube related to the unpopular mobilization. Google said it agreed to remove only one, because the clip advised the use of poison to avoid the draft.

In July, Google prompted ire from the Kremlin when it complied with European Union sanctions on pro-Kremlin musicians and removed their channels and videos. The impeding of service began soon afterward.

Russian authorities have also slapped Google with increasing fines.

Mr. Putin, speaking at his annual call-in show last month, accused YouTube and Google of doing the U.S. government’s bidding by serving up politically oriented videos to Russians searching for culture and music content.

“If they want to work here,” Mr. Putin said, “let them act in accordance with the laws of the Russian Federation.”

Mr. Putin also blamed the disruptions to YouTube last year on Google, saying that the company had not serviced its infrastructure in Russia since retreating from the market. Google denies that technical issues were responsible for the slowdown

Russian authorities have been stepping up a long-running campaign against VPN services, which, if effective, could further reduce Russian access to YouTube and other Western tech platforms.

Apple, for instance, removed scores of VPNs from its app store in Russia last year under apparent pressure from Moscow, a move that outraged international human rights groups. (Google Play, the App Store equivalent for Android devices, which are more popular than iPhones in Russia, has not done so).

Few Russian content creators, including those who support Mr. Putin, are satisfied with being confined to state-controlled domestic YouTube alternatives, which lack the same international reach, recommendation algorithm, monetization possibilities and broad user base.

Mr. Putin’s comments on YouTube in December came in response to a question from a popular Russian-language YouTube blogger, Vlad Bumaga.

Mr. Bumaga, originally from Belarus, praised the Russian alternatives, including VK, which has a deal to air his videos. But he nonetheless asked if YouTube access could remain accessible.

Even after signing with VK, Mr. Bumaga is still uploading his videos on YouTube, where they continue to earn millions of views and thousands of Russian-language comments. His account claims he is based in the United States.

Alina Lobzina and Oleg Matsnev contributed reporting.