LONDON—The first Guantanamo detainee released since President Barack Obama took office returned to Britain on Monday, saying his seven years of captivity and torture at an alleged CIA covert site in Morocco went beyond his “darkest nightmares.”

Binyam Mohamed’s allegations—including repeated beatings and having his genitals sliced by a scalpel—have sparked lawsuits that could ensnare the American and British governments in protracted court battles. Looking frail from a hunger strike, Mohamed, who once was accused by U.S. authorities of being part of a conspiracy to detonate a bomb on American soil, stepped off a charter plane and was whisked away by police, border control agents and immigration officials. The 30-year-old Ethiopian refugee, who moved to Britain as a teenager, was freed after four hours of questioning.

Attorney General Eric Holder, who traveled Monday to Guantanamo Bay as the Obama administration weighs what is needed to shut the facility, thanked Britain for its cooperation in the case.