Russia's media regulator on Monday blocked 49 websites linked to jailed Kremlin critic Alexei Navalny as pressure mounts on opponents of President Vladimir Putin ahead of parliamentary elections.
The blocked websites include Navalny's main website navalny.com as well as the websites of Navalny's regional offices and his key allies.
Navalny associates suggested that the authorities would soon also target a website dedicated to "Smart Voting" -- a proposed strategy to back candidates best placed to defeat Kremlin-linked politicians in elections.
The tactic has seen the ruling United Russia party lose a number of seats in recent local elections.
Russia's opposition says authorities are stepping up a campaign of intimidation against dissenters ahead of a parliamentary vote in September, allegations the Kremlin has rejected.
Some of Putin's most vociferous opponents have fled the country and a number of activist groups and independent media have closed.
Navalny, Putin's most vocal domestic critic, was arrested in January after he returned to Russia following months of treatment in Germany for a nerve agent poisoning he blames on the Kremlin.
In February, he was jailed for two-and-a-half years on old fraud charges and his organisations were in June banned as "extremist", barring members and sponsors from running in parliamentary elections in September.
On Monday, Russian state media regulator Roskomnadzor said that the websites were blocked in accordance with prosecutors branding Navalny's organisations "extremist."
"The mentioned internet resources are used for the propaganda and continuation of prohibited activities of (Navalny's Anti-Corruption Foundation) FBK," the censor said in a statement to AFP.
Navalny's main website was unavailable in Russia Monday.
The opposition politician's allies condemned the move.
"The night of the long knives," top Navalny aide Leonid Volkov wrote on Twitter, in reference to a bloody purge of military and political rivals launched by Adolf Hitler in Germany in 1934.
Volkov, who is living in EU member state Lithuania, added that the group will "soon tell" its followers "how we will get around all this".
"They have decided to completely cleanse us from the Russian internet," added Maria Pevchikh, the head of investigations at the Anti-Corruption Foundation.
Navalny's anti-corruption group has vowed to fight on after being branded "extremist", a move that has been condemned by Western countries.
Russia has stepped up pressure on critics since Navalny's arrest and imprisonment in January, pushing some of the opposition politician's allies to flee the country.
The head of Navalny's fund, Ivan Zhdanov, has been put on an international wanted list and is a target of several criminal probes.
Last week, a Moscow court extended the house arrest of Navalny's spokeswoman, Kira Yarmysh, by six months. She has been under house arrest since February after authorities accused her of breaching coronavirus restrictions at a protest in support of Navalny.
This month a legal group that defended Navalny's organisations said it was disbanding over fears of criminal sanctions.
The Anti-Corruption Foundation routinely published investigations into alleged graft by officials at all levels of government.
Its most notable investigations were YouTube videos showing sprawling luxury properties allegedly belonging to Putin and former president and prime minister Dmitry Medvedev, both of which helped spur mass anti-government protests.