In a statement, Mohamed said: "I have been through an experience that I never thought to encounter in my darkest nightmares.
"Before this ordeal, 'torture' was an abstract word for me. I could never have imagined that I would be its victim.
"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."
Clive Stafford Smith, Mohamed's lawyer and the founder of Reprieve, a UK-based legal action charity, 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."
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".
| Barack Obama has signed an executive order to close the prison at Guantanamo Bay [AFP]
Binyam's sister Zuhra said she was "so glad and so happy, more than words can express".
"I am so thankful for everything that was done for Binyam to make this day come true," she said.
In his statement, Mohamed accused the British government of colluding with foreign governments during his alleged abuse and torture.
He said: "For myself, 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.
"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."
Mohamed originally entered the UK as an Ethiopian refugee when he was a teenager and British authorities say they will review his immigration status when he returns to the country.
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 will be the first of a Guantanamo prisoner since Barack Obama, the US president, who has pledged to close the camp, took office last month.
Mohamed says he was tortured and abused by foreign agents while in custody in Pakistan and later in Morocco, before being transferred to Guantanamo in 2004.
But Washington has denied that he was subjected to "extraordinary rendition", and Morocco said it never held him.
British diplomats and a doctor travelled to Guantanamo Bay more than a week ago to assess Mohamed's medical condition.
He had been on a hunger strike for more than a month and his lawyers were concerned that he could die.
Mohamed was persuaded to end his hunger strike and assessed to be healthy enough to be flown back to Britain.