LONDON (Reuters) - A British resident held at Guantanamo Bay for more than four years returned to Britain a free man on Monday and accused the U.S. government of inflicting medieval torture on him -- with British collusion.

Binyam Mohamed, an Ethiopian citizen with British residency, was returned on a charter flight after the U.S. government agreed last week to a British request to free him. All charges against him were dropped and he was never tried.

He is the first Guantanamo Bay detainee to be freed since U.S. President Barack Obama came to power with a pledge to shut the controversial prison. Human rights groups say 241 detainees remain at the camp, including one British resident.

Mohamed, 30, said he was neither physically nor mentally capable of facing the media, but denounced the U.S. government.

I have been through an experience that I never thought to encounter in my darkest nightmares, he said in a statement issued via his lawyers shortly before his arrival.

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.

Mohamed, who was detained in Pakistan in April 2002 and held for four months, during which his lawyers say he was tortured and abused, was also critical of Britain.

He said that while in detention in Pakistan, he was interrogated and abused by Pakistani intelligence officers in the presence of a British agent. In July 2002, he was taken to Morocco on a CIA plane and again tortured and abused, before being transferred to Guantanamo Bay in September 2004, he said.

Morocco has denied holding him and the United States has denied that he was subjected to extraordinary rendition.

The very worst moment came when I realized in Morocco that the people who were torturing me were receiving questions and materials from British intelligence, Mohamed said.

I had met with British intelligence in Pakistan. I had been open with them. The very people who I had hoped would come to my rescue, I later realized had allied themselves with my abusers.

Britain's attorney general has said she will see if there is enough evidence to order an investigation into the actions of the British government and its agents.

Lawyers for Mohamed said on Monday they could not rule out bringing a lawsuit against the British government, and human rights groups are calling for a public enquiry.

Mohamed was a legal resident when he left Britain and is expected to have that status again now. His lawyer, Clive Stafford Smith, said he would be subject to some controls.

Shortly after his arrival in Britain, police said he had been detained under a part of the anti-terrorism law pertaining to port and border controls but had not been arrested.

Shaker Aamer, a Saudi Arabian with British residency and a British wife, remains in solitary confinement in Guantanamo Bay, where he was been since 2005.