By Amanda Walker, Moscow Correspondent
Investigators in Russia have opened a criminal case against one of the country's most prominent opposition leaders in what is widely viewed as the latest sign of a widening crackdown.
Leftist leader Sergei Udaltsov was accused of plotting mass riots in a recent documentary made by a Kremlin-friendly TV channel.
In a statement, the Investigative Committee said it will look into claims made in the NTV programme that opposition leaders worked with Georgian officials to overthrow the government. Udaltsov himself is not officially suspected of that more serious charge.
Udalstov's profile has grown since mass protests began in earnest last winter.
Investigators and armed men wearing ski masks have raided his apartment in Moscow and his lawyer told journalists that his parent's home was also searched.
NTV, which is seen by many as a propaganda arm of the Kremlin, showed what it claimed is footage of Udaltsov meeting officials from neighbouring Georgia to discuss raising $200m (£120m) to fund anti-Putin riots in Moscow.
Last month's "March of Millions" protest called for Vladimir Putin to quitThe opposition movement on Twitter has reacted strongly, with some saying his treatment is overtly political.
Some tweets drew comparisons with the practice, during Stalin's time, of bringing criminal cases based solely on newspaper articles.
Political analyst Andrei Piontkovsky said this latest episode could become worryingly significant.
"If Udaltsov is formally arrested today, it's a very important event in modern history," he said.
"The charges are completely made up - the evidence wouldn't be sufficient in any normal country with a healthy rule of law. Modern Russia is completely different.
"This move doesn't surprise me, taking in mind all the repressive laws that were implemented during last six months, like fines for participating in unauthorised demos and new laws on libel.
"Currently all opposition leaders like Alexei Navalny, Boris Nemtsov, Ilya Yashin and now Udaltsov have either been charged or are witnesses in criminal cases."
In relation to Udaltsov, the Investigative Committee said it will pursue criminal cases against citizens of Georgia and other unspecified countries.
A spokesman said: "Once their involvement in the preparation of criminal acts is established, they will be subject to criminal liability under Russian law and the norms of international law, and will be issued with international arrest warrants."
Udaltsov, who is currently being detained by police, said he has met "a great number of people" recently to discuss fundraising, but that all of his efforts and intentions are legal.
He insisted the footage presented in the documentary had been doctored, although the Investigative Committee said it had been carefully studied and was genuine.