Russian investigators on Friday filed charges against a left-wing activist, making him the most prominent opposition figure to face prison under an increasingly severe Kremlin crackdown against the anti-Putin protest movement.

Investigative Committee spokesman Vladimir Markin said Friday that Left Front leader Sergei Udaltsov has been charged with plotting riots, a charge that carries a maximum 10-year sentence.

Most observers, opposition activists and even his lawyer expected Udaltsov to be arrested on Friday, but he was released on condition that he not leave Moscow.

"Investigators are playing some sort of a game," Udaltsov tweeted.

A documentary-style program aired earlier this month by a Kremlin-friendly TV channel claimed that Udaltsov and his associates met with a Georgian lawmaker to raise money for organizing riots in Moscow and several other Russian cities. Udaltsov rejected the charges and said the footage was a sham.

Opposition and rights activists have denounced the case against Udaltsov and other activists as a throwback to the times of Soviet-era repression.

Udaltsov and two other defendants have pleaded not guilty. One, Leonid Razvozzhayev, on Thursday retracted a confession, saying he had written it under duress after being kidnapped in Ukraine and tortured for two days.

But Markin said on Friday that Razvozzhayev's retraction does not worry investigators because they have "plenty of evidence" against him anyway.

Razvozzhayev had gone to Kiev last week to seek political asylum in Ukraine. He told Russian human rights defenders who visited him in a Moscow jail earlier this week that he was kidnapped on a street by masked men who smuggled him into Russia.