Police in the Russian capital have arrested at least six protesters as they took part in demonstrations that coincided with President Vladimir Putin's 60 birthday. 

Sunday's protests were held as a demonstration calling for answers in the death of journalist and Kremlin critic Anna Politkovskaya got under way. Politkovskaya was murdered six years ago. 

Police hauled away several protesters outside the offices of Putin in Moscow, including a woman who was bundled away by after she criticised Putin before the media.

The president elebrated his birthday with his family in his hometown of St Petersburg, about 745km northwest of Moscow. 

Putin has been widely criticised for clamping down on dissent during his 12 years at Russia's helm - first as president and prime minister. 

The Associated Press, meanwhile, reported that around 200 people took part in the rally remembering Politkovskaya.

Participants held up candles and pictures of the fallen journalist, a sharp critic of Putin's policies in Chechnya. She was shot inside her apartment building in 2006.

The meeting, attended by numerous opposition figures, was unusual as it had received prior official approval from city authorities.

The US state department said on Saturday it would "continue to shine the spotlight on [Politkovskaya's] case'' and urged more protection for journalists worldwide that speak out against abuses. 

The anti-Putin protest and Politkovskaya rally took place as Russian investigators said they had launched a probe into allegations in a documentary film that claimed an opposition protest leader was seeking to overthrow Putin's government with foreign backing.

The film, "Anatomy of a Protest II", is a sequel to a documentary aired in March that claimed the opposition had bribed people to attend rallies.

The follow-up film - which, like the original, aired on popular television channel NTV - focuses on young radical leftist Sergei Udaltsov, leader of the Left Front movement, a key player in the country's anti-Putin protest movement.

The film claims Udaltsov is planning to overthrow the government with backing from a Georgian lawmaker and former Bank of Moscow president Andrei Borodin, who is in exile in Britain.

It also alleges the Left Front leader is considering hiring Chechen fighters to stage an attack.

Russia's Investigative Committee said it had launched a probe into the film's claims.

"The Investigative Committee is looking into the facts of organisation of mass riots in Russia, as shown in the documentary movie 'Anatomy of a Protest II'," the committee said in a statement late on Saturday.

The film, which aired on Friday, shares the original's jerky camera style and doomsday narration.

"The plan of action has already been worked out. Its authors are abroad," the narrator says, as scenes of clashes between protesters and police during an anti-Putin protest rally fill the screen.

Source: Agencies