A Muslim chaplain who visited Omar Deghayes in Guantanamo Bay has spoken of his horror at the conditions there.
James Yee was locked up himself after complaining about inmates' treatment.
Now he has joined campaigners to call for the immediate release of the remaining prisoners, including Brighton resident Omar Deghayes.
Omar, 38, is expected to be released shortly from the jail in Cuba.
His family were given the impression he would be home for Eid, the Muslim festival, but now are less confident as the Foreign Office has refused to make any official confirmation.
Mr Yee was a United States Army minister who visited about 660 inmates in Camp Delta between November, 2002 and September, 2003.
Omar is believed to have been among them.
Prisoners were held in cages of steel mesh in the open air with a tin roof overhead, a steel bed and a plastic mattress.
Mr Yee said: "The conditions in Guantanamo I often describe as dire and desperate."
He said he saw prisoners being dragged across gravel to interrogation and they often complained of torture. Some were unable to eat because they were too depressed.
Mr Yee said: "There were bruises on prisoners' arms. I spoke with a prisoner who had his tooth knocked out in an interrogation for resisting sexual humiliation.
"They raised their complaints and concerns to me in the hope I would do something about it."
After complaining about their treatment for ten months, Mr Yee was falsely accused of espionage and imprisoned for 76 days on a naval brig. All charges were eventually dropped and he was given an honourable discharge.
Mr Yee said his personal experience of prison had steeled him to fight for justice, truth, diversity, humane treatment and tolerance.
It is believed that Omar has been told about his potential release.
The Foreign Office was invited to send someone to the debate tonight but did not respond.
Jackie Chase, a member of the Save Omar Campaign, said: "We are very frustrated by the British Government."
Sabri Benaneur, vice-chairman of the Brighton and Hove Muslim Forum, said: "The community is absolutely devastated. We have lost trust with the Government."
Abu Baker Deghayes, Omar's brother, said: "We are really confused because at the weekend we were told that Omar is going to be back before Eid."
The last letter from Omar received by any of his family, lawyers or campaigners, came five months ago. His supporters say it was so heavily censored it said barely nothing except "How are you?".
Omar used to write many letters to his supporters and family, but these stopped about 18 months ago.
After his release from Guantanamo Bay, Moazzam Begg was shown a photograph of Omar from his life before imprisonment. Mr Begg took some time to recognise him and told Abu Baker: "I am afraid your brother looks very different now."
Omar is reported to have been badly beaten, with facial injuries and is partially blind after a guard sprayed pepper spray in his already weak eye. He lost a finger after it was crushed in a cell door.
Omar was on hunger strike for 26 weeks and his relatives suspect he was force fed through a nasal tube.
As far as his supporters know, Omar has been in solitary confinement for three years and perhaps has not seen daylight in that time. They have no idea what kind of conditions he is being kept in because they are allowed no contact at all with him.
Mr Begg has said one of the hardest psychological things the prisoners have to deal with is that there have been no charges against them, so they have no case to think about and cannot plan their own defence.
Anyone who supports the Justice for Omar campaign is asked to rally outside Hove Town Hall tonight at 6.30pm, bringing a jam jar and tea light.
There will be a public debate from 7.30pm to 9pm.
Leave your messages of support to Omar's family and friends below.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article