Thousands of protesters risked arrest in Moscow and St. Petersburg on Tuesday, defying a heavy police presence to voice their fury at the sentencing of Alexey Navalny, the anticorruption activist who survived an assassination attempt and then tricked a Russian intelligence agent into confessing that he had poisoned him with a nerve agent.

Despite a security crackdown in the wake of mass demonstrations across Russia since Navalny was arrested, during which thousands have been beaten or detained, an estimated 2,000 protesters marched through central Moscow, chanting for the release of the opposition leader, the resignation of President Vladimir Putin and demanding a “Russia without Putin.”

A feed on the Russian messaging app Telegram called “Live From the Paddy Wagon,” documented the wave of arrests which exceeded 800 in Moscow alone, according to the human rights group OVD-Info.

The Russian news app Baza collected video evidence of police brutality against dissenters, including one cornered group that was beaten while chanting “We are not armed.”

Reporters who filmed the assaults were also clubbed.

The crackdown by the riot police in Moscow had started outside the court even before the verdict was delivered. Among those detained for merely standing there on Tuesday was Dmitry Markov, a popular iPhone photographer who documented his time in the police station on Instagram.

The mass detentions and brutality were clearly intended to act as a deterrent, and many of those arrested at a protest for Navalny on Sunday, including journalists, have been forced to endure harsh conditions in detention.

Reporters outside the country following live streams of the protests on Tuesday night noticed that some men detained as protesters were suddenly released by the police, at least one after uttering what appeared to be a pass phrase likely indicating that he was an undercover agent.

Navalny, whose meticulously researched investigations of corruption allegations against Putin and his allies have enraged the Kremlin, was sentenced to more than two and a half years in a penal colony on the astounding charge of violating his parole by not reporting to the authorities in person while he was receiving life-saving treatment in Berlin following the poisoning.

The European Court of Human Rights previously ruled that Navalny’s original conviction for fraud in 2014, for which he was on parole, was “fundamentally unfair” and politically motivated, since it was intended “to silence a government critic and prevent him from engaging in political activities.” Navalny was barred from running for president in 2018 because of the conviction.

As the Human Rights Watch lawyer Damelya Aitkhozhina noted, Navalny argued in court that, until he was poisoned, he had diligently reported twice a month, in compliance with the terms of his probation, and sent notification of his whereabouts in Germany as soon as he came out of a coma.

As he recovered abroad, Navalny embarrassed the Kremlin by placing a prank call to a Russian intelligence agent who had been identified as his poisoner by the investigative group Bellingcat and tricked the man into detailing the botched effort to kill him. The agent, who thought he was speaking to a superior officer rather than his victim, explained that he had broken into Navalny’s hotel room and coated his underwear with novichok, a neurotoxin developed in the Soviet Union. The same nerve agent was used by Russian intelligence agents to poison a former spy and his daughter in Britain in 2018.

After he was arrested on his return to Moscow last month, Navalny’s anticorruption foundation released a new documentary in which he presented evidence that a $1.3 billion palace intended for Putin’s retirement had been built with pilfered state funds, featuring drone video of the compound and a computer visualization of its lavish interior.

While state-controlled television channels refuse to air Navalny’s charges, the documentary (with English subtitles that can be turned on by clicking the closed-caption icon) has been viewed more than 100 million times on YouTube in just two weeks.

One of the most striking features of the film is the frequently sardonic and relentlessly upbeat tone with which Navalny lays out the case against Putin, refusing to betray any fear, even as the lingering effects of the poison can be seen on his wizened face and in the visible scar on his throat from the ventilator that helped him breathe while comatose.

It was with that same tone that he delivered a scathing denunciation of Putin in a 16-minute statement to the court on Tuesday. Some of his remarks were posted on Twitter with English subtitles by Radio Free Europe/Radio Liberty, a news service financed by the United States Congress to promote democracy abroad.

His arrest, Navalny told the court, was illegal, and entirely motivated by Putin’s “hatred and fear,” according to an English translation of Navalny’s complete remarks by Meduza, a Russian exile news site.

“I mortally offended him by surviving. I survived thanks to good people, thanks to pilots and doctors. And then I committed an even more serious offense: I didn’t run and hide,” Navalny said. “Then something truly terrifying happened: I participated in the investigation of my own poisoning, and we proved, in fact, that Putin, using Russia’s Federal Security Service, was responsible for this attempted murder. And that’s driving this thieving little man in his bunker out of his mind. He’s simply going insane as a result.”

Calling Putin as “just a bureaucrat who was accidentally appointed to his position,” Navalny described the president as a man terrified of facing him in a fair election. “He’s never participated in any debates or campaigned in an election. Murder is the only way he knows how to fight,” Navalny said.

Contrasting Putin with two of Russia’s most famous rulers, Navalny added: “He’ll go down in history as nothing but a poisoner. We all remember ‘Alexander the Liberator’ and ‘Yaroslav the Wise.’ Well, now we’ll have ‘Vladimir the Underpants Poisoner.’”

The activist then urged his fellow citizens to refuse to be afraid of their ruler. “I’m standing here, guarded by the police, and the National Guard is out there with half of Moscow cordoned off. All this because that small man in a bunker is losing his mind,” Navalny said. “The main thing in this whole trial isn’t what happens to me. Locking me up isn’t difficult. What matters most is why this is happening. This is happening to intimidate large numbers of people. They’re imprisoning one person to frighten millions.”

“I hope very much that people won’t look at this trial as a signal that they should be more afraid,” he added. “This isn’t a demonstration of strength — it’s a show of weakness. You can’t lock up millions and hundreds of thousands of people. I hope very much that people will realize this. And they will. Because you can’t lock up the whole country.”

After Navalny was sentenced and taken from the courtroom, Michael McFaul, who served as the United States ambassador to Russia during the Obama Administration, drew attention to a letter his foundation sent to President Joe Biden last week. The letter was sent “to encourage the United States to sanction corrupt Russian allies of President Putin,” and included a detailed list of 35 prominent Russian officials and businessmen.