LONDON (AP) — Dozens of activists have staged a protest outside the U.S. Embassy in London to mark 11 years since the opening of the U.S. prison camp at Guantanamo.

About 50 protesters, some wearing orange jumpsuits and masks of President Barack Obama or a dark sack around their heads, turned out for the peaceful demonstration in central London.

The activists called for the release of Shaker Aamer, the last British resident still held at the prison. Aamer, originally from Saudi Arabia, was detained in Afghanistan in 2001 and has been held at Guantanamo since 2002. He has not been charged and has insisted he was in Afghanistan to do voluntary work.

Obama promised when he took office to close the prison but congressional opposition has prevented him from fulfilling that vow.