Alexei Navalny, the jailed Russian opposition leader, has vowed to fight on despite a court ruling outlawing his entire movement, saying he is supported by millions of Russians who will keep fighting against President Vladimir Putin’s corrupt regime.

The landmark ruling marks the Kremlin’s most daring attempt to do away with any significant opposition ahead of September’s parliamentary elections, where Mr Putin’s ruling United Russia party is expected to struggle to win a majority.

In a hearing that lasted nearly 13 hours and wrapped up close to midnight on Wednesday, a Moscow court upheld a lawsuit that sought to classify Mr Navalny’s Anti-Corruption Foundation and his regional network – the main opposition force in Russia – as an extremist organisation. 

The designation means Mr Navalny and anyone associated with his organisation risks being sentenced to 10 years in jail for “carrying out extremist activity” and puts them on a par with Al-Qaeda.

“We’re not a name or an office. We’re a group of people that brings together and coordinates the Russians who oppose corruption, stand for independent courts and equality in the eyes of the law," said Mr Navalny in a message from prison, where he is currently serving a three and a half year sentence for parole violations.  

“There are millions of us. You are one of them. As long as you’re here, we’re not going anywhere.”

"This is our country and we have no other," the 45-year-old said in an Instagram post.

The Navalny movement said it would appeal the decision.

"We woke up, smiled with destructive intent and knowing that we are a ‘danger to society’ will continue to fight corruption!" the Anti-Corruption Foundation (FBK) wrote on Twitter in response to the late-night ruling from the Moscow city court.

Opposition activist Alexei Navalny attending a hearing via video link while in jail.

Credit: Anna Ustinova /Tass

The extremism case was first announced in April, and Mr Navalny’s vast regional network folded as soon as hearings began.

Several of his staff have since left the country due to pressure and intimidation – but others have sworn to fight on.

“The only way to interpret this ruling is: Vladimir Putin is scared of me and my colleagues,” Violetta Grudina, who led Mr Navalny’s team in the Arctic city of Murmansk, told The Telegraph.

“I’m going to wear this label with pride. I feel nothing but hatred and disgust towards Putin and his accomplices.”

Despite constant harassment from the authorities, Ms Grudina, whose office windows were shot at in April, is running for local council in September as an independent candidate.

Some have even mocked the ruling.

“Woke up an extremist and sat down to work this morning. I don’t feel any different,” tweeted Grigory Alburov, who was behind some of Mr Navalny’s biggest investigations into official corruption.

“I told my wife this morning that I’m an extremist now and I’m allowed to be naughty. She said: ‘No, you’re not.’ What was it all for then?” said Ivan Zhdanov, head of the now defunct Anti-Corruption Foundation.

The European Union on Thursday condemned the court decision, describing it as the latest effort to "suppress" Russia’s opposition.

"It is an unfounded decision that confirms a negative pattern of a systematic crackdown on human rights and freedoms which are enshrined in the Russian constitution," EU foreign policy chief Josep Borrell said in a statement on behalf of the 27 member states.

UK Foreign Secretary Dominic Raab had earlier described the ruling as "perverse" and "Kafka-esque," while the United States called on Moscow to end the crackdown and release Mr Navalny.

President Joe Biden has promised to raise the issue of human rights with Mr Putin when the two meet next week for a summit in Geneva.