image copyrightReutersimage captionAlexei Navalny told his supporters: "We won't step back"
A court in Moscow has banned political organisations linked to the jailed Kremlin critic, Alexei Navalny, classifying them as "extremist".
Activists will risk prison sentences if they continue their work and anyone who publicly supports Mr Navalny's political network can now be barred from running for public office.
Writing on social media, Mr Navalny promised he would "not retreat".
However, he said his supporters would now have to change how they work.
Mr Navalny is in prison for violating the terms of parole in an embezzlement case – charges he says are politically motivated.
Russian parliamentary elections are due to take place in September and opinion polls show the ruling party losing support. Some of Mr Navalny's supporters have been planning to run in the elections.
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Following Wednesday's ruling, a court statement said Mr Navalny's regional network offices and his Anti-Corruption Foundation (FBK) had been banned with immediate effect.
"It was found that these organisations not only disseminated information that incited hatred and enmity against government officials, but also committed extremist actions," prosecutors' spokesman Alexei Zhafyarov said outside the court.
Mr Navalny's lawyers said they would appeal against the ruling.
A message later posted on Mr Navalny's Instagram account said: "We're not going anywhere. We'll digest this, sort things out, change, and evolve. We'll adapt. We won't step back from our aims and ideas. This is our country and we do not have another one."
Paranoia in the corridors of power
The court ruling, in effect, outlaws Alexei Navalny's political movement. With parliamentary elections just over three months away, preventing opponents from taking part seems to be the Kremlin's priority.
That reflects the level of fear and paranoia in the corridors of power here.
With opinion polls showing the ruling party losing support, the Kremlin's worry is that amid economic problems and with no end in sight to the coronavirus pandemic, its political opponents might do well at the ballot box. And that is something the Russian authorities are not prepared to allow.
Mr Navalny – President Putin's foremost critic – was detained in January as soon as he returned to Russia from Germany. He had received treatment in Berlin for a nerve agent attack in Siberia last August that left him in a coma and fighting for his life.
Mr Navalny blamed President Putin directly for the attack which nearly killed him. The Kremlin denied any involvement.
media captionAlexei Navalny: what you need to know
The pro-Putin United Russia party, which dominates parliament, has been described by the opposition leader as "the party of crooks and thieves", and millions of Russians have watched his videos alleging corruption in Mr Putin's elite circle.
Mr Navalny is expected to be a topic on the agenda of talks between Mr Putin and US President Joe Biden at their first summit on 16 June.
Mr Putin, who has been in power as either president or prime minister since 1999, recently said he hoped the Geneva meeting would improve the "extremely low level of relations" between the two countries but expected no breakthroughs.