Clive Stafford-Smith, who visited clients in the US detention camp in Cuba a few weeks ago, told Aljazeera.net that Sami al-Hajj had been beaten by his interrogators.
Al-Hajj, a Sudanese national, was arrested in late 2001 while working for Aljazeera in Afghanistan.
"Sami has endured horrendous abuse - sexual abuse and religious persecution," said Stafford-Smith, who is on a visit to Qatar, on Tuesday.
"He has been beaten. He had a huge scar on his face when I saw him."
The British lawyer said that al-Hajj witnessed the Quran being flushed down the toilet by US soldiers in Afghanistan, and also witnessed expletives being written on the Muslim holy book.
"He is completely innocent. He is about as much of a terrorist as my granddad. The only reason he has been treated like he has is because he is an Aljazeera journalist. The Americans have tried to make him an informant with the goal of getting him to say that Aljazeera is linked to al-Qaida.
"The problem is that nobody is putting pressure on the Americans to release him. One word from the government of Sudan or Qatar would suffice."
"Guantanamo is a PR disaster. It is one of the most iconic symbols of hypocrisy in the world. It is just incredible how the US has squandered all the goodwill it had after the September 11 attacks"
human rights lawyer
Stafford-Smith also said that inmates who were children when they were arrested have been held in Guantanamo Bay since 2001.
Despite the US government's denials, he said they are being held in pitiful conditions.
Moreover, he predicted that the detention centre will soon be closed down.
Public relations disaster
"Guantanamo is a PR disaster. It is one of the most iconic symbols of hypocrisy in the world. It is just incredible how the US has squandered all the goodwill it had after the September 11 attacks.
"What George Bush says about Guantanamo is absolute rubbish. He has never been there himself and he should go there and take a visit to see what it is really like."
The lawyer added: "But we must remember that Guantanamo is just a diversion. It only holds a few hundred inmates while there are around 30 secret US detention centres around the world which hold up to 12,000 inmates. Conditions are worse there than in Guantanamo."
George Bush says Guantanamo
inmates are treated humanely
There are currently more than 500 Muslim men detained at Guantanamo Bay without charge in conditions that have provoked worldwide concern.
Most were arrested in Afghanistan and Pakistan in the aftermath of the 2001 US-led war to topple Afghanistan's Taliban government.
War on terror
Several US politicians, including former President Bill Clinton, have called for the camp to be either "cleaned up" or "closed down".
However, the Bush administration has consistently defended Guantanamo, arguing that most of the people detained there are terrorists who pose a risk to American citizens.
In response to a question about the camp on Monday, US President George Bush said its inmates are treated humanely in accordance with the Geneva Convention.
He said the Red Cross has 24-hour access to the camp and journalists and activists are welcome to visit it to see how it operates.