Masked protesters voicing their support for jailed feminist punk band Pussy Riot have clashed with security forces outside Moscow's main cathedral - where the band claimed global attention by performing an anti-Putin song.

The protest, organised by Amnesty International, featured 18 demonstrators wearing the group's trademark coloured balaclavas and branding a banner comprised of black cards with white letters, spelling out "blessed are the merciful".

The guards arrested four activists, who have been detained and are now likely to face administrative charges.

According to anti-government TV channel Dozhd, the security forces treated protestors roughly when breaking up the demonstration.

This claim appears to be corroborated by liberal daily Novaya Gazeta, which reports that "the guards of the church used physical force, not only on the activists but on journalists covering the protest,"

A citizen journalism website has posted a video in which guards pull away the letters of the message held aloft by protestors, before chasing off both demonstrators and journalists.

Widespread protests

The three members of Pussy Riot - Nadezhda Tolokonnikova, Yekaterina Samutsevich and Maria Alyokhina - have been tried in a Moscow court on charges of "hooliganism motivated by religious hatred or hostility" following their 'punk prayer', which saw the women enter Moscow's Christ the Saviour Cathedral wearing masks, and chant a song imploring the Virgin Mary to "throw Putin out".

The court is due to issue its verdict imminently. If found guilty, the women could face up to three years in prison.

A long list of celebrities such as Madonna, Yoko Ono and Jarvis Cocker have made call for the three bandmates to be freed.

Madonna, who has been cited as an inspiration for the band, added her voice to those of other international artists, saying that what happened to them "is unfair".

Yoko Ono sent a personal message to Yekaterina Samutsevich, saying that "the power of your every word is now growing in us".

Leading British rock musicians such as The Who guitarist Pete Townshend, Cocker, Alex Kapranos of Franz Ferdinand, former Smiths guitarist Johnny Marr, Corinne Bailey Rae and Neil Tennant of the Pet Shop Boys have signed a letter, published in The Times, urging a fair trial - something that seems to have been denied them, say critics.

Protests have already taken place in Iceland, during Reykjavik's annual Gay Pride, and further protests are scheduled outside London's Russian Embassy, the Sagrada Familia Cathedral in Barcelona and the Stavinsky Square in Paris.

Alexey Prokopyev, a 29-year-old activist from Russian-emigrate organisation Russie-Libertés, will lead the Paris demonstration.He told reporters: "Most people go to these rallies in Paris because we cannot be in Russia at the moment for various reasons [such as] jobs, classes.

"We all wish we were in Moscow now, but since we can't we do it in Paris."

The US State Department has expressed concern about what it called the "politically motivated prosecution of the Russian opposition and pressure on those who express dissenting views."