Sir Paul McCartney has added his voice to the growing chorus of music stars supporting a Russian punk rock group that is on trial for staging an anti-Putin protest on the altar of a Moscow cathedral.

In a letter released Thursday, the former Beatles legend called on Russian authorities to release the trio of female rockers from the band Pussy Riot, who've been in prison since March.

"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, according to the Associated Press.

The three women face charges of "hooliganism" stemming from their unauthorized performance in February on the altar of Moscow's Christ the Savior Cathedral in which they asked for divine intervention to rid Russia of Vladimir Putin, who was then running for a third presidential term. They could face up to seven years in prison if convicted. A verdict in the trial is expected on Friday.

Last week, pop diva Madonna took a break from a Moscow concert to call on authorities to release the band.

"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 her performance, according to the The New York Times.

At another point during her concert Madonna donned a wool baklava, Pussy Riot's signature headgear, and later appeared on stage wearing only a black bra and had the band's name written on her back.

Madonna's statements drew a sharp rebuke from Russian Deputy Prime Minister Dimitry Rogozin, who likened her to a former prostitute.

" Every former slut seeks to lecture everyone on morality as she gets older. Especially during tours and gigs abroad," Rogozin wrote on Twitter.

"Either take off your cross, or put on your knickers," he said.

In the past two months, such artists as Sting, the Red Hot Chili Peppers and Franz Ferdinand have also used their Moscow performances as an opportunity to call for the punk band's release.