A British boy whose father has been detained at the Guantanamo Bay camp delivered a letter to Downing St, ahead of a protest outside the US Embassy.
More than 300 protesters gathered outside the US embassy
Anas el-Banna, 10, handed in his fourth letter to Tony Blair, reflecting the years his father had been held.
He was accompanied by MP Sarah Teather, as campaigners marked the fifth anniversary of the camp's opening.
One ex-detainee, Moazzam Begg, said Mr Blair was a "partner in crime" of the US due to his "silence" over the issue.
The demonstration was one of a number organised around the world by human rights group Amnesty International.
A petition was also handed in.
More than 300 protesters gathered outside the US embassy for the hour-long demonstration.
They were dressed in orange boiler-suits, as worn by prisoners in the early stages of the camp, as well as blindfolds, goggles and face-masks.
Mr Begg added: "The greatest travesty of justice in Guantanamo Bay is not the beatings, torture, swearing and the inability to communicate with family.
"It is the illegal limbo you remain in, having no knowledge of when or if you will be charged, when or if you are going to be released and what crime you stand accused of that has earned you five years in the world's most notorious prison."
Plea to Prime Minister
Jamil el-Banna, a UK resident, was on a business trip to Gambia in November 2002, when he was arrested and handed over to the US authorities.
A vigil was held outside Downing Street on Wednesday
Campaigners believe he was then flown to Afghanistan before being transferred to Guantanamo.
Anas' letter read: "I want to know if Mr Blair cares after four years.
"No one knows what the meaning of four years without a dad is and is he still alive? Only God knows."
A candlelit vigil outside Downing Street on Wednesday was intended to put pressure on the government to secure the release of detained UK residents.
Ms Teather, Lib Dem MP for Brent East, said it was "outrageous" it was not doing more.
Amnesty International director Kate Allen criticised the British government's "shameless abandonment" of British residents and urged it to confront the US.
She said: "I don't think terrorism is going to be defeated by the American government abandoning human rights."
Lawyers say Mr al-Rawi is having a mental breakdown
The UK government has already said it would prefer it if the camp was closed.
Mr el-Banna is one of eight British residents with leave to return being held at the camp in Cuba, which has come under fire from human rights groups.
Among them are Iraqi businessman Bisher al-Rawi and his friend, Mr el-Banna, who both lived in London before they were detained.
They have been held without trial in Guantanamo Bay since 2003 on suspicion of having links to terrorism.
Their lawyer, Zachary Katznelson, visited them in November 2006.
He told BBC News that Mr el-Banna was not receiving an appropriate diet for diabetes, which was causing extreme leg pains and a deterioration in his eyesight.
Mr el-Banna has five British children, the youngest of whom he has never met. Mr al-Rawi has been in isolation since March last year and is having a mental breakdown, says Mr Katznelson, of UK charity Reprieve.
Mr el-Banna has never met his youngest child
Both men's MPs are fighting to secure their release.
Liberal Democrat MP Edward Davey asked Prime Minister Tony Blair on 8 January to make a personal plea to US President George W Bush to secure Mr al-Rawi's release.
The Foreign Office said it was in continuing talks with the US over his release and the decision was in the hands of US officials.
However, it said that following a court ruling last July, it was under no obligation to act on behalf of his fellow foreign nationals being held.