According to new reports, Moscow has blocked access to four anti-Putin websites.
Alexei Navalny’s blog, two news sites and one run by Garry Kasparov were blocked on the orders of Russia’s prosecutor general’s office.
In a statement, it said the blocks were imposed because of the sites’ role in helping stage recent illegal protests.
Critics said the blocks were just the latest in a series of moves by Russia’s government to stifle dissent.
The blocks come as Russia’s central bank reported that hackers had targeted its main website. The site, and that of the Kremlin, were briefly unavailable with authorities saying “technical reasons” were making them hard to reach.
Opposition leader Alexei Navalny is currently under house arrest for two months – a punishment imposed for violating the terms of a five-year suspended sentence given after he was tried on charges of embezzlement. The terms of his house arrest demand that he does not receive visitors or use the net.
Anna Veduta, a spokeswoman for Alexei Navalny, said in a tweet that the block was a “political decision taken as part of the cleansing of the media space”.
The block also extends to the Russian news sites Ej.ru and grani.ru as well as the kasparov.ru website.
Alexander Ryklin, editor of the Ej.ru website, said the imposition of the block was “monstrous” and a “direct violation of all the principles of freedom of speech”.
“These sites contain incitement to illegal activity and participation in public events held in violation of the established order,” said a statement from Russia’s state communications monitoring agency Roskomnadzor.
The imposition of the blocks became possible on February 1 after a law drawn up in late 2013 to police sites involved in illegal protests took effect.
It is not clear how successful the blocks will be as followers of Alexei Navalny have widely published details of how to reach the sites and avoid official censorship.