Russian authorities say they have launched a criminal case against opposition leader Sergei Udaltsov, accusing him of planning to ignite mass riots.

Russia's Investigative Committee announced Wednesday that the probe was opened after a television documentary aired on a pro-government television station, appearing to show Udaltsov of conspiring with Georgian officials to overthrow the Russian government. The Committee said the film was examined and no evidence of fakery was found.

Udaltsov has been a prominent figure in recent anti-government protests but denies he has ever plotted violence.

Reports said security officers were searching his Moscow apartment Wednesday.

The Committee's announcement also said it is investigating several other opposition activists, and it included a warning. It said those who think they can organize riots and plan acts of terror while going without punishment are underestimating the professionalism of Russia's security services.

It added that non-Russian citizens found to be plotting against Moscow will be prosecuted under international law.