MOSCOW — Russia placed a leader of mass protest rallies against President Vladimir Putin under house arrest on Wednesday after questioning him in a criminal probe into allegations he was plotting a violent uprising.

The country's Investigative Committee said Sergei Udaltsov, 35, had been "placed under the obligation not to leave his place of residence" -- a form of house arrest under Russian law.

Investigators had earlier questioned Udaltsov, a shaven-headed activist known for his punchy rhetoric and radical politics, after naming him as a suspect in a criminal probe into "plotting to organise mass unrest", said Investigative Committee spokesman Vladimir Markine in a statement.

Udaltsov said he was not guilty.

"I do not consider myself guilty. We are going to work out my line of defence with my lawyer," Udaltsov said after his interrogation, in comments reported by the Interfax news agency.

The Investigative Committee, Russia's top investigative body, brought Udaltsov to its offices for questioning after investigators searched his family apartment for more than five hours accompanied by masked special forces and removed several bags of evidence.

The committee said that before his release Udaltsov had been required to give a written pre-trial undertaking to show good behaviour and not leave his place of residence.

Investigators said another suspect, Udaltsov's aide Konstantin Lebedev, would be detained for a further 48 hours, and that they were seeking another suspect in the case who is an aide to a prominent opposition lawmaker.

The probe comes after a documentary that alleged Udaltsov was plotting a violent uprising against the government aired on a pro-Kremlin national TV channel on October 5.

The film, "Anatomy of a Protest II", claimed Udaltsov was planning to overthrow the government with the help of foreign consultants and Chechen militants.

It claimed Udaltsov was being bankrolled by former Bank of Moscow chief Andrei Borodin, currently in exile in Britain.

In the blurred hidden-camera footage, Udaltsov and his assistants purportedly meet Georgian lawmaker Givi Targamadze and plot a coup in Russia.

The film showed "facts of preparing for mass riots in Moscow and other regions of Russia", the Investigative Committee said in a statement.

The activist has dismissed the film as the "delirium of a lunatic".

"I have never, never in my life met anyone from the Georgian, Israeli or American secret services," he told Russian television.

These are the most serious charges the leader of the Left Front opposition movement has ever faced, even though he is often detained for public order offences at protests.

While Udaltsov has frequently urged protestors to hold peaceful sit-ins, he has denied calling for violent action and said last week the film appeared to be aimed at "preparing public opinion for new arrests".

The Investigative Committee has in the past come under fire for being overly politicised in its probes -- an accusation that Putin's opponents reiterated Wednesday.

"The case against Udaltsov is absolutely absurd. The fabrication is obvious, since it is based on nothing but edited film," anti-corruption crusader and fellow protest leader Alexei Navalny told the Moscow Echo radio station.

Udaltsov rose to prominence during the winter protests against Putin's 12-year political dominance, and has been one of the key speakers at opposition rallies.

Thirteen people are currently under arrest for alleged mass rioting during an opposition protest on May 6, with four more barred from leaving Moscow during the investigation.

Putin returned to the Kremlin for a third term in May against the backdrop of an unprecedented wave of protests against his rule.