Sir Paul McCartney has thrown his substantial artistic credibility behind Russian punk rock group Pussy Riot, joining a long list of musicians vocally supporting the band as it stands trial for staging an anti-Putin protest in Moscow.

"I and many others like me who believe in free speech will do everything in our power to support you and the idea of artistic freedom," McCartney said in a letter released Thursday.

The three female rockers are charged with "hooliganism" after staging an unauthorized performance on the altar of Moscow's Christ the Savior Cathedral in February. A verdict is expected Friday. They could be sentenced up to seven years in prison if convicted.

The trio asked for divine intervention to help Russia oust Vladimir Putin, who was running for his third term as president.

"I would like you to know that I very much hope the Russian authorities would support the principle of free speech for all their citizens and not feel that they have to punish you for your protest," McCartney wrote. "Many people in the civilized world are allowed to voice their opinions and as long as they do not hurt anyone in doing so I believe this is the best way forward for all societies."

McCartney's show of support follows Madonna's call to release the band during a concert in Moscow.

"I know there are many sides to every story, and I mean no disrespect to the church or the government. But I think that these three girls - Masha, Katya, Nadya - I think that they have done something courageous. I think they have paid the price for this act. And I pray for their freedom," Madonna said during the concert.

Other bands, including Franz Ferdinand, Sting and the Red Hot Chili Peppers have used stage time in Moscow to call for the bands release.

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