In a new sign of a widening Kremlin crackdown on the opposition, investigators on Wednesday opened a criminal probe against leftist leader Sergei Udaltsov and several other activists for allegedly planning mass riots.

Russia's top investigative agency said it also will investigate claims made in a recent documentary aired by a Kremlin-friendly TV channel that opposition leaders worked with Georgian officials to prepare terrorist attacks across Russia.

Udaltsov, 35, a leather-clad, shaven-headed leader of the Left Front opposition movement, has denied the charges stemming from the documentary, which he said was a sham.

The agency did not say whether Udaltsov and others had actually carried out any of their alleged plans.

The criminal case comes as the opposition is bracing up for an online election of its coordination council this weekend, in which Udaltsov is running.

Udaltsov, who wore a Stalin T-shirt for his wedding, has led anti-Putin protests for several years, focusing on unsanctioned marches and rallies.

A great-grandson of a Bolshevik revolutionary, he has been arrested more than 100 times during his political career and spent months in prison. He has launched many hunger strikes while in custody.

Investigators, backed by armed men wearing ski masks, searched Udaltsov's apartment in south Moscow for more than five hours on Wednesday. The home of his parents was also searched, said Violetta Volkova, Udaltsov's lawyer.

"I'm going to hold on until the end, and I won't be quiet," Udaltsov said as he left his home for questioning escorted by masked armed police. "It's a wave of new repression."

In addition to Udaltsov, the criminal investigation is targeting two other leftist activists, one of whom, Konstantin Lebedev, was detained for 48 hours following brief questioning, Volkova said.

After several hours of questioning, Udaltsov was ordered not to leave Moscow pending the probe.

The documentary aired last week on NTV, a channel seen as a propaganda arm of the Kremlin, showed what it says was footage of the Left Front leader meeting with officials from neighboring Georgia to discuss raising funds for protests against President Vladimir Putin, and plans for organizing mass riots.

In late 2011 and early this year, a series of massive peaceful demonstrations against Putin were held in Moscow that drew more than 100,000 people in the largest show of discontent against the Kremlin since the 1991 Soviet collapse.

The Investigative Committee said that it would pursue criminal cases against not only Russians but also citizens of Georgia and other unspecified countries.

"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," the committee said.

Officials in Georgia have refrained from commenting on allegations of Georgia's involvement with the Russian opposition. Givi Targamadze, a senior Georgian lawmaker who was featured in the NTV program as the mastermind of Georgian support, was not available for comment on Wednesday. However, he told Georgian media last week he had never met Udaltsov.

Relations between Russia and Georgia have remained frozen since a brief 2008 war.

Udaltsov said he has met "a great number of people" recently to discuss fundraising, but all of his efforts and intentions are legal. He has insisted the footage presented in the documentary has been doctored.

The Investigative Committee said Wednesday that it had carefully studied the footage and concluded it was not tampered with.

Renowned human rights activist Lev Ponomarev told the Interfax news agency on Wednesday that a "broad crackdown on the opposition is very dangerous for this country" and said that early morning searches reminded him of secret police tactics in the 1930s in the Soviet Union.

The Russian Communist Party, which forms the largest opposition faction in parliament, has supported Udaltsov, dismissing allegations against him as nonsense. Party leader Gennady Zyuganov said Udaltsov is being persecuted for his views.

"The main goal is to nip the protests in the bud," he told Interfax.


Nataliya Vasilyeva and Mikhail Metzel from Moscow, Misha Dzhindzhikhashvili from Tbilisi, Georgia, and Peter Leonard from Almaty, Kazakhstan, contributed to this report.