Russian authorities blocked access to jailed Kremlin critic Alexei Navalny's website on Monday in the run-up to a parliamentary election, along with those of 48 other individuals and organisations affiliated with him.
A Russian court last month approved a prosecutor's request to declare organisations linked to Navalny as extremist, in effect outlawing them and preventing his allies from taking part in September's election to the State Duma lower house.
The United States and Britain condemned the court's ruling as an unfounded blow to the Russian political opposition.
Russian Internet regulator Roskomnadzor said in a statement to Reuters on Monday it had moved to block navalny.com, the banned movement's main website, and others inside Russia, at the request of the prosecutor general.
It said the sites helped the movements covered by the court ban to distribute propaganda and continue their illegal activities.
Condemning the move, Navalny's team said on social media it expected the authorities would soon target its so-called smart voting website, which advise peoples how to vote tactically to try to unseat candidates from the ruling United Russia party.
Leonid Volkov, a Navalny ally, said he and others would soon explain what people should do to try to avoid website blocks.
Navalny is President Vladimir Putin's most prominent domestic critic and is serving a two-and-a-half year jail sentence for parole violations that he says were trumped up. His jailing has increased strains in Russia's relations with the West, which has called for him to be freed.