A criminal case has been opened against the leader of the Russian opposition's Left Front movement, Sergei Udaltsov, and several other opposition leaders.
A spokesman for Russia's Investigative Committee, Vladimir Markin, said the case centered on a film called "Anatomy of a Protest 2" and added that the charge was preparing for mass riots.
Markin also said opposition leaders were working with Georgian officials in a plot to overthrow the Russian government.
Udaltsov was released from detention late on October 17, and emerged accusing authorities of carrying out "revenge against all of us for our activity during the last months," when protests have targeted the return to the presidency of Vladimir Putin and perceived abuses of what detractors label "Putinism."
The pro-Kremlin television channel NTV aired the film last week, claiming Udaltsov met with officials from Georgia to plan riots in Moscow.
Some of the other leaders who allegedly appeared in the film and were named in the criminal case were Leonid Razvozzhayev and Konstantin Lebedev. Markin said the authorities were searching their flats.
The flats of Udaltsov and his parents were also searched.
Markin said "experts" had established that a voice heard in the film was the voice of Udaltsov.
The Investigative Committee said the film also showed Udaltsov meeting in Minsk with the chairman of the Georgian parliament's Committee for Defense and Security, Givi Targamadze.
It said the meeting was also attended by the Georgian consul to Moldova, Mikhail Iashvili, and focused on raising funds for protests against President Vladimir Putin and organizing riots in Moscow.
The film also purportedly shows Udaltsov plotting to hang anti-Putin banners on Moscow's iconic Ivan the Great Bell Tower.
Udaltsov denies those accusations and said the film in question was doctored.
Markin said experts determined the video was authentic.
"As the result of the investigation, no traces indicating that the video was doctored have been found. It was established that the voice on the video shot -- including the footage made by a surveillance camera -- and used in the film 'Anatomy of Protest 2' belongs to Udaltsov," Markin said.
"The meeting itself, fragments of which were shown in the video, took place in the second half of June 2012 in a residential apartment in the capital of Belarus, Minsk."
Udaltsov made a brief statement to reporters as masked members of Russia's special forces escorted him away for questioning.
"This is all lawlessness and repression. I hope society will not be silent about it," he said.
Udaltsov was released after questioning on October 17 but had travel restrictions imposed on him.
Investigative Committee spokesman Markin said Udaltsov was freed after he signed a pledge not to leave Moscow.
He denied all the charges against him.
"I categorically denied these accusations," Udaltsov said. "I've said that they had nothing to do with reality, that they all came out of the imagination of a sick mind, and I want to stress once more that I had no contacts with any western special services, with the Georgian intelligence. I haven't received any instructions nor money from them nor could I have done, since, although we are the opposition, we use peaceful methods and we do not support violence and bloodshed."
Earlier on October 17, reports said Udaltsov would be held for 48 hours.
Investigators said Lebedev had also been detained, while Razvozzhayev's whereabouts remained unknown.
Opposition leader Aleksei Navalny told the Ekho Moskvy radio station that the case against Udaltsov was "absolutely absurd" and a "fabrication."
Lyudmila Alekseyeva, the head of the Moscow Helsinki Group, said Udaltsov was unlikely to be the only opposition leader or rights activist to be facing charges.
"It depends on the degree of fear on the part of authorities and on the degree of their vengeance," Alekseyeva said.
Lev Ponomaryov, the leader of the For Human Rights movement, said the actions against Russia's opposition "remind one of the start of the repression in the 1930s," when Soviet leader Joseph Stalin initiated the great purge.
With reporting by Interfax, ITAR-TASS, and AFP