The two stars are among those supporting Lead Voice Project's new Imprisoned for Art project, created to raise funds and awareness for those who have been locked away for upsetting politicians and leaders with their artistic expression.

Pussy Riot star Nadya Tolokonnikova and rocker and activist Tom Morello have also joined the initiative, with each celebrity posing for a 'mug shot' photograph linked to an imprisoned artist.

Gabriel holds a slate that reads "Amsara Eritrea, Indefinite Detention" - a reference to writer Dawit Isaak, who was been held in prison in Eritrea since 2001 without trial, while Depp pays tribute to Ukrainian filmmaker Oleg Sentsov, currently imprisoned in Russia on suspicion of plotting terrorist acts.

Each photograph will be printed on a T-shirt and proceeds from sales will benefit Lead Voice Project.

Announcing his part in the campaign, longtime human rights activist Gabriel says, "When I first started traveling around the world, I was shocked to discover in how many countries there were artists who were in jail, or who had been tortured or killed for doing exactly the same thing that I do - writing and singing songs

"We have to defend and protect those with the courage to speak out."