It is not hot stabbing pain that Omar Deghayes remembers from the day a Guantánamo guard blinded him, but the cool sen­sation of fingers being stabbed deep into his eyeballs. He had joined other prisoners in protesting against a new humiliation – inmates ­being forced to take off their trousers and walk round in their pants – and a group of guards had entered his cell to punish him. He was held down and bound with chains.

"I didn't realise what was going on until the guy had pushed his fingers ­inside my eyes and I could feel the coldness of his fingers. Then I realised he was trying to gouge out my eyes," Deghayes says. He wanted to scream in agony, but was determined not to give his torturers the satisfaction. Then the officer standing over him instructed the eye-stabber to push harder. "When he pulled his hands out, I remember I couldn't see anything – I'd lost sight completely in both eyes." Deghayes was dumped in a cell, fluid streaming from his eyes.

The sight in his left eye returned over the following days, but he is still blind in his right eye. He also has a crooked nose (from being punched by the guards, he says) and a scar across his forefinger (slammed in a prison door), but otherwise this resident of Saltdean, near Brighton, appears ­relatively ­unscarred from the more than five years he spent locked in Guantánamo Bay. Two years after his release, he speaks softly and calmly; he has the unlined skin and thick hair of a man younger than his 40 years; he has just remarried and has, for the first time in his life, a firm feeling that his home is on the clifftops of East Sussex.

Deghayes must, however, live with the darkness of Guantánamo for the rest of his days. There are reminders everywhere, from the beautiful picture of Saltdean that was painted for him while he was incarcerated, to the fact that Guantánamo ­remains open 12 months after Barack Obama vowed to close it within a year.

There are still around 200 prisoners left in the detention camp, many of whom have been there for eight years. Of the 800 freed, only one has been found guilty of any crime and he was convicted by a dubious military commission, a verdict that is likely to be overturned. Deghayes, too, does not want to forget. He says there is so much still to be ­exposed about the ­conditions there, and about British ­collusion in the ­extraordinary rendition and torture of men such as him in the months following the American-led ­invasion of Afghanistan in 2001.

Deghayes, one of five children of a prominent Libyan lawyer, first came to Saltdean from Tripoli aged five, to learn English with his brothers and ­sisters on their summer holidays. He would return and stay with British families every summer. Then, in 1980, his father, an opponent of the increasingly totalitarian Gaddafi, was taken away by the authorities. Three days later, Deghayes' uncle was told to ­collect his body from the morgue. ­Harassed and increasingly fearful for their safety, Deghayes' mother sought asylum for her family in Britain. They settled in the place they knew best, Saltdean, in a large white house with fine views over the sea. More than two decades on, the family still lives there.

After a secular upbringing in ­Saltdean, Deghayes became a practising Muslim while at university in ­Wolverhampton, where he graduated in law. When he finished studying to become a solicitor, he had a "longing" to return to Libya but couldn't because of his family name and opposition to Gaddafi, so he left for a round-the-world trip to ­experience Arabic cultures and visit university friends. He enjoyed ­Pakistan's mixture of west and east, and was then tempted into a trip to ­Afghanistan: he saw business oppor­tunities and the chance to use his ­languages (Farsi, Arabic and English) and legal training (understanding both western and Sharia law) to help ­import-export companies.

He fell in love with the country and an Afghani woman; they married and had a son. "I liked the country – such beautiful rivers and different terrains. The people were difficult to get to know at first, but if they knew you and liked you, they'd open their hearts and houses to you," he says. Afghanistan, it seems, triggered many ambitious dreams: he says he helped set up a school in Kabul, assisted NGOs, ­experimented with an agricultural ­social enterprise and exported apples to Peshawar. "I was generating income for myself but I had more ambition than that – to establish myself as a ­lawyer," he says. "Things were really good. Then this war broke out and ­everything was shattered."

Fearing for his new family's safety, he paid people-smugglers to get them all back to Pakistan in early 2002 after the US-led invasion of Afghanistan. He hoped his mother would take his wife and child back to England, while he planned to return to Afghanistan and continue his NGO and legal work. "I still thought I had nothing to fear. Even if there was an invasion, there was nothing I had been doing that was illegal."

They rented a house in Lahore, "far away from the war atmosphere". But then the Americans began paying large amounts of money to find Arabs who had been in Afghanistan. Suddenly, he was lucrative bounty for the Pakistani authorities. "The atmosphere changed completely. Nice Pakistan turned into a trap," he says. One day, their house was surrounded by armed police. He was seized, but not taken to a ­normal police station. Instead he was driven, fast and under heavily armed guard, between secure rooms in hotels and villas. A Kafkaesque nightmare had begun.

Deghayes says he was beaten and ­interrogated first by Pakistani officials. He thinks the Americans and the ­Libyans competed to "buy" him from the Pakistanis, and it appears the Americans won: when he was moved from Lahore to Islamabad, a man ­introduced himself as the head of the CIA's Libyan section. Taken between hotels by armed guards, Deghayes ­believes he saw a man who is now listed as a disappeared prisoner: an Italian Moroccan. "I remember seeing him; he was with me in the same car in Islamabad. He came out crying from the meeting, scared; he was saying, 'No, don't do this to me.'"

Deghayes also describes meeting a British interrogator when he met the CIA section head for the second time. "I was facing the British man, who introduced himself as Andrew. He spoke in an obvious British accent." According to Deghayes, Andrew said he was from the intelligence services and wanted to question him.

"I was really annoyed and said, 'You shouldn't do this, you're helping these people – I'm kidnapped, abducted against my will. Your job is to get me out of here. I'm British and if I go back to England, I will take you to court for what you are doing now.' Andrew was a little bit scared, but he looked at me and said, 'What case would you bring against me?' I had nothing in my mind. He said, 'Listen, if you answer my questions and co-operate with me, I will do my best. I will get you out of there.'"

Deghayes was shown an ­album of 100 photographs of supposed terrorists. He says he did not recognise anyone. One morning, he was tied, bound and blindfolded and taken to an airport. The "thin black bag" was removed from his head: he was standing in front of a mirror, guarded by two US soldiers. They tied another bag over his head, which "felt worse than the first bag – it suffocated me." It smelt "like socks or cheese," he says. "This was an indi­cation of the new regime – there were even harder times coming up."

Inside the plane, it was mayhem: his feet and hands bound together and covered in bags, Deghayes was bundled on top of others in the hold. "People were crying. People were throwing up. Some people were suffocating, and there was a kick here and a kick there: 'Get your head down, you bastard!' Things like that. Then the plane took off and you could smell [the guards] drinking spirits."

They landed in what he later ­realised was Bagram military air base. Here, Deghayes' clothes were taken away and he was given two pieces of blue uniform. He was not allowed to speak to fellow inmates, and was bound to barbed wire before, he says, being beaten and made to suffer "all sorts of humiliation". He spent several months there. "There were no rules in Bagram; people just went in and kicked people if they didn't like them."

He says he did not eat for more than 50 days. "I was really sick; I became a skeleton. I couldn't walk any more. I lost my mind – I was really scared for my mental safety. I tried to eat but I threw up. I started to hear voices in my head because of the hunger. People would say something and I could not understand what they were saying. You hear shouts and you're speaking to yourself inside your head. I started to become really scared because I thought I was losing my brains and ­going crazy."

While he was in Bagram, he was again interrogated several times by ­officials he believes were from Britain. "They felt I was lying to them. I said to them I studied in ­Holborn, London. They said, 'Which train did you take to get there?' They didn't believe anything," he says. "They weren't free to do what they liked; the Americans were running the show." When he said he was too sick to speak, they called him "a bandit".

His British interrogators "came up with lots of ­stupid things" – suggesting the scuba­diving lessons he had taken in the shabby lido in Saltdean, within yards of his family home, were terrorist training. "The Americans took that up in Guantánamo. It was a big headache. They showed me books of military ­scubadiving and ships and mines and they said, 'Which ones did you see?'" The British also accused him of teaching people to fight in terrorist training camps in Chechnya, and claimed they had secret video evidence.

Deghayes had never been to ­Chechnya, and thought all these allegations ­laughable. Only later did he discover through Clive Stafford Smith, director of the human rights charity Reprieve, that his apparent appearance in an ­Islamic terrorist training video in Chechnya was the crucial evidence in a flimsy case against him. The ­authorities refused to give Stafford Smith, who campaigned for Guantánamo detainees, a copy of this videotape, but he eventually obtained one through the BBC.

It was, says the Reprieve director, an ­obvious case of mistaken identity: the person depicted lacked Deghayes' small childhood scar on his face. ­Stafford Smith was able to show that the videotape was of a completely different ­person, actually a Chechnyan rebel called Abu Walid, who was dead. "This was typical of the whole Guantánamo experience," says Stafford Smith. "They said they had evidence and they wouldn't let you see it. Then when you did, it was incorrect."

After two months in Bagram, Deghayes was flown to Guantánamo in autumn 2002. There, prisoners were treated brutally. According to Deghayes, when guards physically subdued them by tying them down, they would "do actions to pretend as if they are raping you. They put you down on your stomach. It was really horrible, all sexual and psychological stuff." On other occasions, he says, guards would hold a prisoner's head and "bang it on the floor".

Deghayes developed a personal ­policy of resistance. Guards would ­typically arrive at a prisoner's cell and spray pepper and other chemicals through the "bean-hole", the hatch in the door. While most prisoners cowered at the back of their cell, Deghayes says he would grab the guards' hands and attack them. He fought back, as viciously as he could, trying to take the fights with guards out of the privacy of his cell and into the corridors.

"It was chaos; they would fall on top of each other and it was embarrassing [for them]. They were wearing all this heavy stuff [body armour] which didn't help either," he says. Some guards ­became afraid of going into his cell. Most, he says, were Puerto Rican and were not driven by the patriotism of the "war on terror". They did not want to get hurt for their meagre wages.

Deghayes did not realise how badly his eye had been beaten until a year ­after the incident, when he looked in a mirror for the first time in four years. He accepts his resistance caused him more physical pain, but believes it ­subsequently helped him. In the camp, he was less fearful.

"I was targeted more, but I was also relaxed compared with others who didn't do that. It was really scary for [the guards] to come into my cell," he says. "Being humiliated by getting beaten up is better than giving your own trousers out. If I'd done those things, I would've been really bitter now. I'm probably less bitter than ­anyone else because I know I gave them a really hard time. If I had given in, and all this was bottled up, I would have been like I see them [other ex-prisoners] – really bitter, full of hatred."

Deghayes says his suffering made his faith stronger; it helped him ­survive. "We knew there's a Muslim [God] ­behind things, there's a hereafter, our patience and hardships will be ­rewarded and the pain has to end sometime. Our religion teaches these things – the good always prevails and the bad is only temporary; the patience of Job, the patience of Moses. All these teachings make a difference." Praying five times a day delivered ­transcendence, removing him from the material world of bodily suffering. "My body and physical being can be chained, can be tarnished, can be beaten, can be raped," he says now, "but not the spiritual: that is something that nobody can bind down. The spirit is what makes us who we are."

As a campaign to free him gained momentum back in Brighton, Deghayes languished in Guantánamo for nearly six years. He was never charged or convicted of anything, by any authority. "And never been apologised to either," he adds. Finally, in August 2007, the British government requested the ­release of Deghayes and four other ­detainees who were legal British ­residents. In the month before his ­release in December 2007, he says, he was deliberately fed well so he would not emerge looking gaunt and half-starved. "For one month we were ­fattened up with milk shakes, ­chocolates and really good cakes."

When he returned to his family in ­Saltdean, he was happy but also dis­orientated. "You know if you are in a forest or walking on the moon, you can't tell what is what. I was like this when I came out," Deghayes says. He was stunned by some of the changes in ­Britain. "To my shock, when I came out from prison the whole country had changed – the surveillance, the Islamophobia, the control orders, secret ­evidence, and people being under ­curfews not being able to leave the house." His neighbourhood also ­appeared to have altered: "We never had thugs and mobs in the street ­before, and kids didn't go binge-drinking or stealing. When I came back, these were some of the changes that I had to adjust to," he says.

While he is very appreciative of the support he had in Brighton, after he was freed his family was targeted by racist teenagers who bullied his ­nephews and threw stones and bottles at their house for months. This stopped, abruptly, after a community meeting and media coverage led the police, rather belatedly, to install a video camera in the window of their home.

His imprisonment also caused his marriage to break down. His wife wrote to him in prison but her letters were never delivered; nor were his to her. "It's cruel, isn't it? These were just ­normal letters between husband and wife." Both believed they had abandoned each other, and they divorced. She now lives with her family in ­Afghanistan. His son, Sulaiman, who is now eight, is staying with Deghayes' mother in the Emirates. They hope eventually to bring him to Britain and give him a western education.

Two years after he was released, Deghayes remarried in ­December and is now busy buying furniture for a new place in Brighton. "Brighton is such a nice city. You can just walk by the sea, and the fresh air comes across. It ­reminds me of Tripoli. ­Before, I used to long for Tripoli; now, only recently, I have started to prefer Brighton. Maybe when you are younger you want to go back to dreams, and when you get to 40 you start to think, this is nicer, this is really what I like."

Deghayes now works with ­Reprieve and other survivors of Guantánamo on legal challenges, ­including a civil case being brought against the Home Office with help from Gareth Peirce, the human rights lawyer. Deghayes hopes there will be a public inquiry into Guantánamo to bring those to account who were ­involved in his interrogation. Financial damages are not, he says, his ­motivation. "Even if I get damages, I will give them to ­charity. The court is an opportunity to embarrass and ­expose those who committed these crimes."

While Reprieve campaigned to get Deghayes released, Stafford Smith ­explains how Deghayes "was a ­tremendously helpful ally in Guantánamo because he was fluent in English and he had a bit of legal training". Stafford Smith brought him legal textbooks but they were censored as a "threat" to national security, and he says he worried for Deghayes' safety during his incarceration. "If it had been me, I would have taken the course of quieter resistance. I was always afraid for Omar, that he would get himself beaten up. I was concerned for him ­because he was constantly being beaten up by the guards, but there's nothing you can do to stop Omar loudly saying what is just and right."

Stafford Smith believes Deghayes has fared better than many veterans of Guantánamo since his release because he had the support of his family, an ­education – and because he has taken a very positive approach to his experiences. "He's not just sat back and taken it; he's tried to do something positive. Omar works a lot with us to try to help other prisoners who are still in Guantánamo. He's also always been up for a good argument or a good ­debate."

Deghayes appears remarkably calm; but his brother, Abubaker, says he has noticed signs of trauma. "His memory is not as good as it was. He forgets to switch off lights. If he opens a window, it stays open. He stays up at night a lot, thinking." Abubaker is not surprised his brother struggles to sleep. "Imagine the lights are on for six years." Has Deghayes changed as a person? "A lot of the things Omar had in his character seem to have deepened, like rebellion and resistance and not accepting oppression. I think they became more rooted in him rather than being beaten out of him."

But isn't he ever tempted to retreat to a quiet place, start his own business, and ­renounce the ­hassles of political campaigning? "I don't want that life," Deghayes says firmly. "I never ­intended to live like that before imprisonment, and nor do I intend that after imprisonment. I would not be true to ­myself if I did.

"Life is worth more. It's good to be a number in society rather than a zero. There are many zeros around but every ­human is ­worthy of being a number, and I hope I will be something of a change for the good, rather than for harm and wars. I hope so. I really hope so."