Russia's media watchdog has said it is slowing down the speed of Twitter, accusing the US social media company of failing to remove 3,000 posts relating to suicide, drugs and pornography.
The move was "to protect Russian citizens", Roskomnadzor said.
Twitter is widely used by the Kremlin's opponents and Russian users said they were having difficulty accessing photos and videos on the site.
As the decision came into effect, the watchdog's website went down.
Media watchdog officials said the disruption, which affected a number of Russian websites including the Kremlin, was unrelated to the action against Twitter and involved technical issues at Russian state internet provider Rostelecom, which was also affected.
The watchdog said it was reducing the speed of Twitter on all mobile phones in Russia and on half of desktop devices. It cited Twitter's failure to remove "banned" content that it said incited the suicide of minors and contained indecent images of children, as well as information on drug use.
There were reports that internet connection speeds in general had slowed down.
Twitter is the sixth biggest social media site in Russia and widely used by opposition figures including Alexei Navalny, who was jailed in January on his return to Russia after treatment for a poisoning attack in Siberia.
When big rallies took place across Russia over his detention, the media watchdog warned Twitter, TikTok, Facebook and other sites that fines would be imposed if posts urging young people to protest were not deleted. Earlier this month Russian authorities said they were suing Twitter and four other social media companies for allegedly failing to delete such posts.
How Russia can slow down Twitter
By Cristina Criddle, BBC Technology Reporter
This is the first time that the Russian government has flexed its muscles in this way, using laws signed in 2019 that gave authorities the power to restrict access to US social media sites.
Internet service providers use network equipment, called deep packet inspection (DPI), which enables the government to track and filter content. Roskomnadzor will compel these providers to slow down the speeds of Twitter for users.
Officials have previously tested a "sovereign RuNet" - an independent network that routes the country's web traffic and data through state-controlled points. This means the Kremlin can block Russian connections to websites around the world or slow down the flow of data for Russian users accessing certain sites.
Supporters say it offers protection if the West ever tries to sever Russia's internet access. But activists say it gives the Kremlin the power to censor Russians from the rest of the world.
'No desire to block anything'
President Vladimir Putin recently gave the media watchdog the power to block social media platforms if they "discriminated" against Russian media. In a speech in January he accused social media giants of "de facto competing with states", playing an increasing role in the life of society.
Roskomnadzor warned Twitter in its statement that if it failed to delete posts containing "illegal" material then it could be blocked entirely.
It cited content containing indecent images of children; inciting the suicide of minors as well as information on drug use.
"Roskomnadzor has filed over 28,000 preliminary and repeated orders to delete unlawful links and publications," the statement said, adding that 3,168 remained unblocked.
Kremlin spokesman Dmitry Peskov said there was "no desire to block anything, but it is quite reasonable to take measures to force these companies to comply with our laws".
The main goal, he told a daily briefing, was for Russians to have access to all global resources, as long as those resources stayed within the law.
There have been many satirical memes about Russia's announcement, shared in a range of languages. Some show Russian President Vladimir Putin as a superhero, such as Spiderman or Ironman, saving the world.
One of the most popular being shared portrays Mr Putin riding a bear with a giant syringe strapped to his back.
A version of this meme even claims, incorrectly, that it was shared by Mr Putin himself.
Another widely shared post celebrating Russia's vaccine announcement appeared on a Facebook page supporting Mr Putin, and has been shared more than 200,000 times.
It's not an official account of the Russian president, who is believed to shun social media.
But many of the tens of thousands of people commenting in numerous languages on the post clearly believe it is.
The viral post compares the first Soviet satellite which "paved the way for humanity in space" with the Russian vaccine which "will pave the way to the future without Covid-19, masks and social isolation."
Increasing international concern that safety may have been compromised in developing a vaccine has led the World Health Organization (WHO) to urge Russia to follow international guidelines.
A reputation for exaggeration
Another post that also proved popular on Facebook claims that Russia has "repeatedly saved the world with its vaccines".
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