Omar Deghayes, who fled to Britain from Libya in 1986, lost sight in one eye after guards put pepper spray in both his eyes and gouged one eye socket, human-rights lawyer Clive Stafford Smith said on Thursday.
Smith has previously represented British nationals at the US base in Cuba, the last four of whom were released in January, also alleging mistreatment after being freed.
Deghayes' mother, Zohra Zewawi, wept as Smith described at a London news conference the alleged abuse meted out when guards entered his cell in March 2004. "They brought their pepper spray and held him down," he said.
Eyes forced open
"They held both of his eyes open and sprayed it into his eyes and later took a towel soaked in pepper spray and rubbed it in his eyes," Smith said.
US army officials deny torturing
Guantanamo prison inmates
"Omar could not see from either eye for two weeks but he gradually got sight back in one eye."
Smith added, "He's totally blind in the right eye. I can report that his right eye is all white and milky - he can't see out of it because he has been blinded by the US in Guantanamo."
Smith, who did not give a motive for the alleged assault, said one of the guards also pushed his finger into Deghayes' eye, helping to cause the loss of sight.
The 35-year-old Deghayes came to Britain from Libya in 1986, six years after his father was allegedly killed by the government of Libyan leader Muammar al-Qadhafi. He has the right to residency in Britain and has applied for full citizenship, his family said.
"I can't believe
you can hold someone for three years in such terrible conditions without coming up
Campaigners have urged the British government to push for the release of up to seven British residents who do not hold British passports still held at Guantanamo Bay. Ministers, however, say they can do nothing for non-nationals.
Deghayes' mother said it was "laughable" to expect the Libyan government to push for the release of someone who had fled the country so long ago. He had been held at the base since early 2002, she added.
"I can't believe you can hold someone for three years in such terrible conditions without coming up with evidence," she said.
A series of British nationals freed from Guantanamo Bay have alleged they were tortured. US officials have consistently denied mistreating prisoners at the base.