A Russian opposition leader's assistant is facing criminal charges by an investigative agency.
The Investigative Committee has said in a statement that Left Front member Konstantin Lebedev was charged with plotting mass riots and could face a jail term of up to 10 years.
Investigators said the 33-year-old Lebedev, who has been in custody since Wednesday, has denied the charges and refuses to give evidence.
A court sanctioned his arrest on Thursday.
On Wednesday, officials interrogated Lebedev's boss, Sergei Udaltsov, who is also a suspect, and searched his home before releasing him on the condition that he not leave Moscow.
Investigators are relying on hidden camera footage aired earlier this month by a Kremlin-friendly TV channel.
The documentary claimed that Udaltsov met officials from the neighbouring nation of Georgia 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.
Mass riots alleged
Investigators said Udaltsov, Lebedev and another leftist activist who remained at large, Leonid Razvozhayev, are suspected of planning to stage mass riots involving the use of firearms, ransacking and arson in Moscow, Kaliningrad, Vladivostok and other 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.
"I think that one thing is really evident: the goal to arrest Udaltsov was set, and in order to achieve it this film or programme was fabricated."
Russian investigators said they had carefully studied the footage and concluded it was genuine.