In a new sign of a widening crackdown on Russian opposition, investigators on Wednesday opened a criminal probe against leftist leader Sergei Udaltsov and several other opposition activists for their alleged role in plotting mass riots.

The Investigative Committee said in a statement it will investigate claims made in a recent documentary aired by a Kremlin-friendly TV channel that opposition leaders worked with Georgian officials to overthrow the government.

Udaltsov has been one of the most recognizable faces of last winter's anti-government protest in Moscow.

Investigators, backed by armed men wearing ski masks, searched Udaltsov's apartment in Moscow. The home of his parents was also searched, said Violetta Volkova, Udaltsov's lawyer.

A documentary aired last week on NTV, a channel seen as a propaganda arm of the Kremlin, showed what it says was footage of Udaltsov meeting with officials from neighbouring Georgia to discuss raising $200 million for protests against President Vladimir Putin, and plans for organizing riots in Moscow.

Udaltsov said he has met "a great number of people" recently to discuss fundraising, but all of his efforts and intentions are legal. He has insisted the footage presented in the documentary has been doctored.

The Investigative Committee said Wednesday that it had carefully studied the footage and said it was not tampered with.

Renowned human rights activist Lev Ponomarev told the Interfax news agency on Wednesday that a "broad crackdown on the opposition is very dangerous for this country" and said that early morning searches reminded him of secret police tactics in the 1930s in the Soviet Union.

The Russian Communist Party, which forms the largest opposition faction in parliament, has supported Udaltsov, dismissing allegations against him as nonsense.