Russia said on Wednesday it was disrupting Twitter's services because the platform had failed to remove "illegal" content, the latest in a series of moves exerting control over foreign tech giants.
In recent months, the Kremlin has been clamping down on sites including Facebook, Twitter and YouTube for hosting content supporting jailed opposition politician Alexei Navalny.
Roskomnadzor, Russia's media watchdog, said the disruption aimed to "protect Russian citizens" after Twitter failed to comply with its requests to delete content related to child pornography, drug use and calls for minors to commit suicide.
The watchdog did not reference calls to join opposition protests demanding Navalny's release that had angered officials earlier this year.
Roskomnadzor said the restrictions would amount to a "slowdown in service speed" for all mobile users and 50 percent of desktop users, later adding it would only affect photo and video content.
Officials in January accused foreign internet companies of interfering in Russia's domestic affairs over their failure to take down calls to participate in rallies in support of Navalny.
President Vladimir Putin then warned against the increasing influence of large tech companies, saying they were "competing" with sovereign states.
The watchdog warned that if Twitter ignored Russian law it could face further restrictions, including a complete ban.
Later Wednesday, Twitter said it was "deeply concerned by increased attempts to block and throttle online public conversation."
The social media giant said it did not support "any unlawful behaviour".
"We have a zero-tolerance policy regarding child sexual exploitation, it is against the Twitter rules to promote, glorify or encourage suicide and self harm," a spokesman added.
Kremlin spokesman Dmitry Peskov insisted Russia had "no desire to block anything" and said the measures forcing companies to comply with Russian laws were "quite reasonable".
According to tech monitoring website Downdetector, there was a spike in disruptions reported by Russian Twitter users after the measures were announced.
- Facebook 'next' -
Lawmakers welcomed the regulator's decision and warned that other social networks could soon be targeted.
"I am sure Twitter will lose big money," said Anton Gorelkin, a lawmaker on a parliamentary telecommunications committee.
He reportedly said Facebook could be the "next candidate" for restrictions.
Another lawmaker, Alexander Bashkin, said the decision would be "sobering" for YouTube and other platforms.
Moscow also recently raised concerns about the Chinese video-sharing app TikTok that was flooded with calls to demonstrate in support of Navalny in January.
The Russian government has spent years tightening its control over the internet in the name of fighting extremism, terrorism and protecting children.
Officials have repeatedly fined Google for failing to remove content and last year fined Twitter and Facebook for refusing to store the personal data of Russian citizens on local servers.
A 2019 law proposes a "sovereign internet" aimed at isolating the country online, a move activists fear will tighten government control of cyberspace and stifle free speech.
Mikhail Klimarev of the Internet Protection Society said Wednesday that Russia was using Twitter to test "repressive laws" and that it was "obvious" Facebook and YouTube would be next.
The head of the Roskomsvoboda digital rights NGO, Artyom Kozlyuk, said the authorities were using children's protection as a cover to "limit access to political and opposition content" on social media.
The measures "inevitably violate constitutional rights", he told AFP.
Moscow has already banned a number of websites that refused to cooperate with authorities, such as video platform Dailymotion and professional networking website LinkedIn.
Russian authorities also attempted to block the Telegram encrypted messaging service, but lifted restrictions after their measures failed to fully cut off service.