Mohamed said in a statement: "I have been through an experience that I never thought to encounter in my darkest nightmares.
"It is difficult for me to believe that I was abducted, hauled from one country to the next, and tortured in medieval ways, all orchestrated by the United States government."
"The very worst moment came when I realised in Morocco that the people who were torturing me were receiving questions and materials from British intelligence," he said.
"I had met with British intelligence in Pakistan. I had been open with them. Yet the very people who I had hoped would come to my rescue, I later realised, had allied themselves with my abusers."
Speaking to Al Jazeera, Ahmed Ghappour, a human rights lawyer from Reprieve, a UK-based legal action charity, said Mohamed was suffering mental instability after having had razor blades taken to his penis at least once a week and being threatened with rape and electrocution in Morocco.
He said that Mohamed, who had recently been on hunger strike, was 50 pounds underweight.
'Powerful' probe call
David Miliband, Britain's foreign minister, said: "I am pleased that Binyam Mohamed has today returned to the UK following his release from Guantanamo Bay.
"This is the direct result of our request for his release and return, and follows intensive negotiations with the US government."
Ghappour said the UK government should initiate "an independent investigation to get behind what's gone on so that it can never happen again".
"I'm confident that the UK government will acquiesce to all of these conditions and do what's right," he said.
Welcoming the British government's statement that it was glad to see Mohamed released, Ghappour said: "We hope there's 100 per cent truth to it."
Mark Seddon, Al Jazeera's correspondent at RAF Northolt, said: "Binyam says questions were being supplied to his Moroccan torturers by British intelligence officers.
"The big question is was this a freelance operation, or did people know further up in the food chain? That's why [there is] this very powerful call ... for an independent inquiry."
| Barack Obama has signed an executive order to close the prison at Guantanamo Bay [AFP]
British and American lawyers are suing to have access to what they claim are secret documents proving the US sent Mohamed to Morocco where he was tortured and also proving that Britain knew of the mistreatment - a violation under the 1994 UN Convention Against Torture.
Britain's attorney general has opened an investigation into whether there was criminal wrongdoing on the part of Britain or a British security agent from MI5 who interrogated Mohamed in Pakistan, where he was arrested in 2002.
Clive Stafford Smith, Mohamed's lawyer and the founder of Reprieve, said he hoped the British government would "allow Binyam's immediate release".
Stafford Smith said: "He is a victim who has suffered more than any human being should ever suffer.
"He just wants to go somewhere very quiet and try to recover. Every moment that he is held compounds the abuse he has endured."
Washington has denied that Mohamed was subjected to "extraordinary rendition", and Morocco said it never held him.
'Glad and happy'
In a statement, Reprieve said Mohamed said he wished to thank "all those in Britain who have worked for his freedom, including many members of the British government".
Binyam's sister Zuhra said she was "so glad and so happy ... [and] thankful for everything that was done for Binyam to make this day come true".
Mohamed originally entered the UK as an Ethiopian refugee when he was a teenager and British authorities say they will review his immigration status following his return.
The decision to release Mohamed follows a formal request by London to Washington in August 2007, asking for the remaining five British residents held at the camp on Cuba to be released.
The transfer is the first of a Guantanamo prisoner since Barack Obama, the US president, who has pledged to close the camp, took office last month.
Ghappour, who represents clients in Guantanamo, said the abuse there had got worse since the Obama administration had come to power.
Eric Holder, the US attorney general, travelled to Guantanamo Bay on Monday, as the Obama administration weighed what was needed to shut the facility.
Meanwhile, two senior British judges have reopened a case into whether 42 secret US intelligence documents shared with Britain should be made public.