Russian opposition activist Alexei Navalny has been sentenced to three-and-a-half years in prison in a case widely seen as an attempt to neutralise President Vladimir Putin’s most prominent critic.

In a tense hearing in Moscow’s Simonovsky court on Tuesday that intermittently descended into bouts of quarrelling between the legal teams, the judge agreed with prosecutors who alleged that Mr Navalny had violated the terms of a suspended sentence for fraud he received in 2014, by not immediately returning to Russia from Germany after recovering from a nerve agent poisoning last autumn.

The judge said that the close to a year already served by Mr Navalny under house arrest would be deducted from the time to be served in the prison colony. 

Mr Navalny, 44, whose corruption investigations and protests against Mr Putin’s regime have become the biggest political threat to the Kremlin, had argued he was unable to attend mandatory judicial meetings while ill because he was incapacitated, and lambasted the court as a tool of political oppression.

“The main goal of this trial is to scare a huge number of people. That’s how it works. They jail one person to scare off millions,” Mr Navalny said in a fierce speech to a room packed with journalists and diplomats.

He claims the charges against him are Mr Putin’s revenge for surviving the poisoning — which several western governments have blamed on the Kremlin — and exposing a hit squad that tracked him around the country for years, then allegedly poisoned his underpants with the nerve agent novichok. The Kremlin denies involvement.

“However much [Mr Putin] tries to portray himself as a geopolitician, a great world leader, he resents me because he’ll go into history as a poisoner. There was Alexander the Liberator and Yaroslav the Wise, and he’ll go into history as Vladimir the Underpants Poisoner,” he said.

Mr Navalny’s jailing demonstrates the Kremlin’s increasing resolve to clamp down on dissent, analysts say. Although constitutional changes last year potentially allow Mr Putin to extend his 20-year rule until 2036, his ratings have approached record lows among Russians unhappy with stagnant real incomes and Moscow’s handling of the coronavirus emergency.

As he and his lawyers engaged in at times aggressive and testy arguments with prosecutors, police outside the court arrested 354 of Mr Navalny’s supporters, according to OVD-Info, an NGO that tracks arrests.

“I really hope people won’t take this trial as a sign they need to be more scared. The [guards] and the cage are a demonstration of weakness. You can’t jail millions of people,” Mr Navalny told the court.

His arrest immediately after he flew back to Moscow last month sparked the biggest protests in modern Russian history. The demonstrations in more than 120 cities nationwide tapped into popular anger about corruption, injustice and plunging living standards. Police arrested more than 10,000 people over the past two weekends and some face charges for offences such as blocking traffic and protesting illegally.

Police detain a Navalny supporter outside Moscow’s Simonovsky court during the hearing into whether the opposition leader violated the terms of a 2014 suspended sentence © Evgenia Novozhenina/Reuters

The prosecution case centred on claims that Mr Navalny missed judicial meetings mandated under the suspended sentence while in Berlin recovering from the attack and could have returned to Russia as soon as he was discharged from hospital.

“I was in a coma,” Mr Navalny said in a raised voice when prosecutors accused him of “hiding” from them in Berlin. “On what grounds are you saying you didn’t know where I was? You’re misleading the court,” he added.

Since he was detained last month, Mr Navalny has used his online following to bypass a near-total media blackout and mobilise his supporters against Mr Putin. A video he released the day after his arrest detailing a lavish $1.3bn palace on the Black Sea allegedly built for Mr Putin has racked up more than 100m views on YouTube.

Mr Putin later said the palace did not belong to him or any of his family members, but did not address Mr Navalny’s claims that his billionaire friends managed a beneficial ownership scheme on his behalf. One of them, his childhood judo sparring partner Arkady Rotenberg, has said he is the palace’s owner and that it is intended to be a luxury apartment hotel.

The Kremlin has used unprecedented force to break up the recent protests, shutting large sections of central Moscow and St Petersburg and paralysing the work of Mr Navalny’s foundation, including by arresting staff and opening legal actions against them.

Mr Navalny’s brother Oleg — who spent three-and-a-half years in prison on the same fraud charges — and several of his top aides were placed under house arrest last week on charges of violating public coronavirus health guidelines by calling for the protests.

EU foreign policy chief Josep Borrell has vowed to call for Mr Navalny’s release during his visit to Moscow on Thursday.

But the Kremlin — which claims Mr Navalny is a CIA stooge bent on bringing Russia to ruin — has shown little sign it will be swayed by international criticism. 

Though the European Court of Human Rights threw out Mr Navalny’s “arbitrary and manifestly unreasonable” conviction in 2017, last year’s constitutional changes included a proviso allowing Russia to ignore international rulings if it deemed them to contradict its own law. Mr Putin also recently ordered legal officials to explore setting up a Russian human rights court that could replace the ECHR.

For Mr Navalny, his legal travails may have only just begun. He also faces up to 10 years in prison on separate charges of embezzling from his foundation, which he denies. But in court, he remained unbowed.

“When injustice and lawlessness are wearing your uniforms and passing themselves off as the law, every honest person’s duty is to resist you and fight you with everything they’ve got,” he told the court.