Russian opposition activist Sergei Udaltsov was charged with plotting riots on Friday, and could face up to 10 years in prison if convicted.

The criminal charges are a sign that the Kremlin is determined to keep up its crackdown on an increasingly bold dissent movement in Russia. Udaltsov is the third high-profile activist to be investigated in connection with a controversial documentary that purports to show Russian opposition figures plotting with Georgian nationals to stoke riots in Russia.

Udaltsov, 35, is a leading figure in Russia’s Left Front movement, an activist political coalition that advocates for socialism in Russia. He has also served as the leader of the Vanguard of Red Youth, a Communist organization. He has been a critic of Russian President Vladimir Putin for years.

Authorities began investigating the activist this week after a documentary was broadcast on a government-run television channel. The film allegedly shows Udaltsov -- along with fellow activists Konstantin Lebedev and Leonid Razvozzhaev -- working with Georgians to fund and plan disruptive activities and protests in Russia.

Udaltsov was briefly detained on Oct. 17, and his apartment was searched. He spoke dismissively about the documentary and its implications.

“I categorically deny receiving money sponsored from abroad, not to mention preparing any violent acts,” he said earlier this month, according to Russian news outlet Ria Novosti.

But Russian authorities have already used the documentary as a basis to investigate not only Udaltsov, but also Razvozzhaev and Lebedev.

Razvozzhaev is currently being held in a Moscow prison. Russian officials say he had turned himself in, but the activist claims he was seeking asylum at a United Nations office in Kiev, Ukraine, on Friday when Russian authorities seized and tortured him, eventually forcing him to sign a paper confessing that he had worked with Georgian nationals to plan protests in Russia.

Razvozzhaev reappeared in Moscow on Sunday. As he was being escorted to a police car, Russian TV crew filmed him as he yelled about being kidnapped and tortured.

That incident provoked a response from the U.S. Embassy in Moscow; spokesman Joseph Kruzich said on Thursday that he was “deeply concerned” and urged the Russian government to investigate the issue thoroughly.

Meanwhile Lebedev, who was detained on Oct. 17 along with Udaltsov, was charged the next day and has never been released from custody.

Udaltsov was unexpectedly released after his detention, on the condition he remain in Moscow. He has since been updating his Twitter account with posts about the investigation.

“Investigators are playing some kind of game,” he tweeted on Friday.

He also reported that he made his way to be the investigative committee offices on his own, rather than being escorted in hadcuffs.

“The Putin regime is on trial here,” he posted. “Come to the [investigative committee] offices to support me and other political prisoners!”

In pursuing the charges against Udatlsov, Lbedev and Razvozzhaev, the Kremlin risks enflaming an opposition movement that has grown increasingly bold this year.

In March, thousands rallied to protest Putin's election to a controversial third term as president, with some international observers calling his victory illegitimate.

This summer, two high-profile cases of dissent made international headlines. In July, three members of the punk band Pussy Riot went on trial after being arrested for "hooliganism"; they had criticized Putin in a performance at a Moscow cathedral. Two of them were sentenced to serve two years in a brutal penal colony -- this month, they launched an appeal and lost.

In July, human rights activist Alexander Navalny, was charged by state investigators with embezzlement that allegedly occurred years ago. Navalny could face a decade in jail; he maintains that the charges are ridiculous and politically motivated. The investigations are ongoing.

The Kremlin has been criticized for this apparent crackdown, but officials maintain that they are simply enforcing existing laws.

Udaltsov has not yet been arrested and remains free to roam the streets of Moscow, though he is under heavy surveillance. His supporters suspect that the Kremlin’s plan is to allow the activist to flee, thereby depriving the Left Front of a charismatic leader.

"If somebody expected me to run across the border like a scared dog, they will not see this," said Udaltsov on Friday, according to the BBC. "I have not committed any crime."