Paul McCartney has promised to do everything in his power to help jailed rockers Pussy Riot.

Three stars of the Russian band are behind bars awaiting a court verdict on charges of hooliganism after they performed a protest song against President Vladimir Putin at a church in Moscow earlier this year.

Singers including Madonna and Bjork have already thrown their weight behind a campaign to free the trio, and now Beatles legend McCartney has joined them, insisting jailing the musicians impinges on their "artistic freedom".

In an open letter to the caged rockers, posted on his official website, McCartney writes, "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..."

"I hope you can stay strong and believe that 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."