A rally calling for an immediate end to the detention without trial of 13 terror suspects is taking place at London's Belmarsh prison.
Protesters said detainees were held in "concrete coffins"
Around 150 people, including civil rights campaigners, gathered outside the prison gates from midday on Sunday.
Campaigners have described the prison as "Britain's own Guantanamo Bay".
The men are being held under emergency powers in the Anti-Terrorism, Crime and Security Act, which was introduced in the wake of the 11 September attacks.
Campaigners held placards saying "Belmarsh is Britain's Guantanamo" and distributed leaflets claiming the detainees were being held in "concrete coffins" in
conditions which were "tantamount to torture".
A number of groups were represented at the demonstration, including the Campaign Against Criminalising Communities, the Muslim Parliament of Great Britain, the Green Party of England and Wales, and the Stop the War Coalition.
Among the speakers was civil rights lawyer Gareth Peirce who spoke of the "mental distress" those being held had suffered due to their indefinite detention.
She told the crowd: "The detainees feel completely forgotten - thank you for showing them they are not."
ANTI-TERRORISM ACT: KEY ELEMENTS
Foreign terrorist suspects can be detained indefinitely without charge, if deportation is not possible
Police have power to stop and search suspects, whether or not they believe they hold articles which could be used for terrorist acts
Unlike old terror laws, MPs need not approve the new legislation annually - it will remain on the Statute Book unless repealed
Any hope the captives had at the beginning of their detention had "now ebbed away", she said.
"They don't know the evidence against them, they just know they are locked up. The hearings are held in secret and the lawyers are not told either."
Deputy Mayor of London and Green Party MP Jenny Jones told the rally it was a "complete injustice" that people should be held without trial in a country which prided itself on its democracy and justice system.
And Dr Ghayasuddil Siddiqui, the leader of the Muslim Parliament of Great Britain, said the detention of innocent men was intended to give the false message to the world that Muslims were the "enemies of civilisation".
Home Secretary David Blunkett has said he believed the ability to detain foreigners without trial "was, and remains, necessary".
Sunday's protest featured live performances and music, while messages of solidarity were gathered to be given to the prisoners.
A two-minute silence was also observed to show support for the detainees.
Ms Peirce has recently expressed anger over the treatment of Muslim detainees, and has warned that the country had entered "a new dark age of injustice".
Dr Ghayasuddil Siddiqui said the detention gave a false message about Muslims
The protest follows the release of a Libyan man - known only as "M" - who was detained at Belmarsh under anti-terror laws for 15 months.
Britain's most senior judge, Lord Chief Justice Woolf, upheld a previous ruling that his detention had been unjustified.
The judge said Mr Blunkett had not established "reasonable suspicion" that M had al-Qaeda links, although he accepted some evidence raised suspicions.
The home secretary had earlier appealed against a decision by the Special Immigration Appeals Commission (SIAC) that M's detention was unjustified.