MOSCOW — Russia's top investigative agency filed criminal charges Thursday against an opposition leader's assistant, continuing a widespread crackdown on the movement against President Vladimir Putin.

The Investigative Committee said in a statement that Left Front member Konstantin Lebedev has been charged with plotting mass riots and could face a jail term of up to ten years.

Investigators said the 33-year-old Lebedev, who has been in custody since Wednesday, denied the charges and has refused to give evidence.

Officials interrogated Lebedev's boss, Sergei Udaltsov, who is also a suspect, on Wednesday and searched his home before releasing him on the condition that he not leave Moscow.

Investigators are relying on hidden camera footage aired by a Kremlin-friendly TV channel.

The documentary claimed that Udaltsov met with Georgian officials to raise money, partly from disgraced banker Andrei Borodin, to overthrow Putin's government by sending communists and neo-fascists to seize the Kremlin as well as several Russian cities.

Udaltsov, a shaven-headed communist firebrand who wore a Joseph Stalin T-shirt to his own wedding, has called the documentary "filth" and said the charges are revenge for his political activity.

He insisted that the TV footage, in which the figure purported to be him is nearly always out of focus, was doctored.

Both Udaltsov and Givi Targamadze, the senior Georgian lawmaker that Russian officials say is behind the plot, deny ever meeting each other.

Russian investigators said they had carefully studied the footage and concluded that it was genuine.