MOSCOW (AP) — In a new sign of a widening crackdown on Russia’s opposition, investigators on Wednesday opened a criminal probe against leftist leader Sergei Udaltsov and several other activists for allegedly plotting mass riots.

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

Udaltsov, a 35-year-old, shaven-headed Communist who wore a Stalin T-shirt for his wedding, has been one of the most recognizable faces of last winter’s anti-government protests in Moscow, which were legal and peaceful.

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 little known Russian leftist activists, but their role in the alleged plot was not identified. One of his allies, Konstantin Lebedev, was detained for 48 hours following brief questioning, Volkova said. Udaltsov’s questioning was still ongoing in the late afternoon on Wednesday.

A 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 $200 million for protests against President Vladimir Putin, and plans for organizing riots in Moscow.

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. Lawmaker Givi Targamadze, who was featured in the NTV program as the mastermind of Georgian support, was not available for comment on Wednesday but told Georgian media last week that he had never met Udaltsov.

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 said 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. "There’s no one left in the Kremlin who can say ‘no’ to that."

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