Moscow (AFP) - Armed police on Thursday raided the office of a Russian rights group led by exiled Vladimir Putin critic Mikhail Khodorkovsky, during the president's marathon annual phone-in session.

A group of police and special forces searched the Moscow office of Open Russia, a rights network led by Khodorkovsky, for protest flyers, Khodorkovsky's spokeswoman Kulle Pispanen wrote on her Facebook page.

"As we're preparing for the phone-in, masked men came to the office," Pispanen wrote. "They are looking for posters and flyers for a protest on April 19 that we were not planning to participate in and which has been cancelled."

Moscow authorities have refused permission for an opposition march on April 19, although activists said they may hold solo pickets instead.

Open Russia is Khodorkovsky's network of activists and journalists which aims to discuss alternatives to Putin's rule and is heavily critical of his policies.

The organisation is run out of central Moscow while Khodorkovsky himself lives in Switzerland following his release from prison after a presidential pardon from Putin in late 2013.

Open Russia posted a scan of the search warrant on its website. Police said they received a tip-off that the group was producing "materials containing extremist calls" ahead of the planned protest.

The warrant authorised police to seize electronic equipment and accounting documents from the organisation.

Khodorkovsky however said the real reason was a documentary the group is working on about the relationship between the Kremlin and Chechen leader Ramzan Kadyrov, known for his heavy-handed governing of the Caucasus region.

"The real reason for the search in Open Russia is the making of a film about Kadyrov's place in the current power structure," Khodorkovsky said in a comment posted by the group.

"The raid is happening as Putin is talking about the 'usefulness of opposition's participation in the elections'," Khodorkovsky added incredulously.

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Employees of Open Russia wrote on social networks that the search was conducted by seven anti-extremism officers and 10 riot police armed with automatic rifles who were not letting them make phone calls.

"They are seizing everything that can carry data: computers, hard drives, flash drives," wrote employee Veronika Kutsyllo posting a picture of stacked hard drives on Twitter. "Looks like those are the orders."

The search began just as Putin was starting his annual televised question and answer session, which usually lasts for several hours and dominates all media coverage.

Khodorkovsky, once Russia's richest man who owned oil company Yukos, spent 10 years in prison over two controversial convictions for embezzlement and fraud, which his supporters said were politically motivated.