Shaker Aamer, 43, has been held without charge for almost nine years at the United States' detention centre in Cuba.
He was captured in December 2001 by the US, who allege he is a Taleban fighter. He was moved to Guantanamo in February 2002.
Saudi-born Aamer, formerly resident of London, led the first hunger strike at Guantanamo and was one of the detainees who claimed he was tortured numerous times in Afghanistan, including by US officials while British personnel were present.
Around 70 supporters of the Save Shaker Aamer Campaign gathered at the proposed site of the new US embassy in London to hold a rally and demonstration - part of a day of events to highlight his case.
Spokesman Ray Silk said: "We want to publicise the terrible plight of Shaker Aamer. He has been incarcerated for nearly nine years in Guantanamo Bay. He has not been charged with anything or brought to trial.
"We are calling upon the UK and US governments to make arrangements as soon as possible for his release. We understand both governments want him to be released and the British government wants him back in the UK."
A Foreign Office spokeswoman said: "The Foreign Secretary (William Hague] raised the case of Shaker Aamer with US Secretary of State Hillary Clinton during his visit to Washington in November and was told that the US government continues to consider the case.
"As the Prime Minister (David Cameron] has made clear to Parliament, the government continues to make best efforts to secure Shaker Aamer's release and return to the UK. Ultimately this is a matter for the US."