Russian authorities say they have blocked a website linked to Alexei Navalny, which told citizens how to vote against pro-Kremlin candidates.
The state communications watchdog, Roskomnadzor, announced the move just weeks before Russia's parliamentary elections.
The decision comes after months of efforts by Moscow to neutralise the imprisoned opposition leader and his supporters.
In June, a Russian court outlawed Navalny's Foundation for Fighting Corruption and a network of his regional offices as extremist organisations.
The ruling barred people associated with the groups from seeking public office and exposed them to lengthy prison terms.
In response, Navalny urged Russian citizens to visit his Smart Voting website for instructions on how to vote against Kremlin candidates.
The website — developed by Navalny's team in 2018 — aims to identify candidates who are in the strongest position to defeat those from United Russia, the party that dominates the parliament and is President Vladimir Putin's power base.
This "Smart Vote" strategy did have some success in local elections since 2019, notably in Moscow.
Roskomnadzor last week warned Google and Apple could face fines if they didn't remove the Smart Voting app from their stores.
And on Monday, the authority said it had blocked the website because it was being used to "continue the activities and holding events of an extremist organisation".
The upcoming election on September 19 is widely seen as an important part of Putin's efforts to cement his rule before Russia's 2024 presidential vote.
The Russian leader, who has been in power for more than two decades, pushed through a constitutional reform last year that would potentially allow him to hold onto power until 2036.
Navalny, Putin's most determined political foe, was arrested in January upon returning from Germany, where he spent five months recovering from a nerve agent poisoning that he blames on the Kremlin. Russian officials have denied any involvement.
In February, Navalny was ordered to serve two-and-a-half years in prison for violating the terms of a suspended sentence from a 2014 embezzlement conviction that he dismissed as politically motivated.