Russia has said it is slowing down the speed of Twitter in retaliation for its alleged failure to remove banned content, and threatened a total block if the US platform did not comply with its deletion demands.
The move, which escalates a growing stand-off between Moscow and US social media firms, comes weeks after Russian authorities accused Twitter and others of failing to delete posts it said illegally urged children to take part in anti-Kremlin protests.
Twitter did not immediately respond to a request for comment.
Russia has traditionally taken a more hands-off role in policing the internet than neighbouring China.
But as domestic political tensions have risen this year over the arrest and jailing of Kremlin critic Alexei Navalny - which triggered nationwide protests - it has signalled a tougher line.
Roskomnadzor, the state communications regulator, said in a statement that as of Wednesday there were more than 3,000 posts containing illegal content on Twitter, which it accused of ignoring its deletion requests for years.
Twitter was already under pressure in Russia after it was named as one of five social media platforms being sued for allegedly failing to delete posts urging children to take part in illegal protests, the Interfax news agency cited a Moscow court as saying on Tuesday.
The regulator did not mention content related to opposition protests in Wednesday's statement, but referred to what it said was illegal content on Twitter containing child pornography, information about drug abuse and calls for minors to commit suicide.
"The slowing down will be applied to 100% of mobile devices and on 50% of non-mobile devices," the regulator said.
"If (Twitter) continues to ignore the requirements of the law, the enforcement measures will be continued... (right up to blocking it)," the regulator said.
The move would affect video and photo content but not text, Interfax cited a regulatory official as saying.
'Control the information space'
The Kremlin said there was no desire to block content but that companies had to abide by the law.
Some activists, however, said they believed the curbs were linked to recent protests.
"Of course the main motive is the increase in street protest action," said Sarkis Darbinyan, an internet freedom advocate with the Roskomsvoboda group.
"It's 10 years since the Arab spring this year... they've understood the internet is a driving force. Any desire to control the Russian internet is connected to the desire to control the information space."
Navalny's allies say they plan new protests in coming months.
Some government websites were unavailable for some Russian internet users shortly after the announcement about Twitter.
Telecoms operator Rostelecom said the disruption to several government websites, including those of the Kremlin and parliament, was not caused by new restrictions imposed on Twitter but by an equipment malfunction.
Tougher internet laws
Vadim Subbotin, an official at the watchdog, said it was possible that the authorities could target and slow down other internet platforms if they failed to comply with the law, the Interfax news agency reported.
Parliament's lower house in December backed big new fines on platforms that fail to delete banned content and other legislation that would allow them to be restricted if they "discriminate" against Russian media.
Moscow has gradually introduced tougher internet laws in recent years, requiring search engines to delete some search results, messaging services to share encryption keys with security services and platforms to store user data on servers in Russia.
Some of those measures have spurred fears of China-style Internet curbs, but have only been partially successful.
Russia tried to ban the Telegram messenger service in 2018 for example, but proved technically unable to block the app and last year publicly lifted the ban.