Amnesty International said the often harsh and inhumane conditions at the camp were "pushing people to the edge".
Many of the 385 inmates have been held for five years or more, unable to mount a legal challenge to their detention.
"While the United States has an obligation to protect its citizens... that does not relieve the United States from its responsibilities to comply with human rights," the report said.
"Statements by the Bush administration that these men are 'enemy combatants,' 'terrorists' or 'very bad people' do not justify the complete lack of due process rights," the group said.
Amnesty reiterated its call for detainees at the prison camp in Cuba - many of whom are suspected Taleban and al-Qaeda fighters - to be released or charged and sent to trial.
'Already in despair'
The report, published on Thursday, said about 300 detainees are now being held at a new facility - known as Camp 5, Camp 6 and Camp Echo - comparable to "super-max" high security units in the US.
It said the detainees were reportedly confined to windowless cells for 22 hours a day, only allowed to exercise at night and could go for days without seeing daylight.
The organisation's UK director, Kate Allen, described the process at Guantanamo as "a travesty of justice".
"With many prisoners already in despair at being held in indefinite detention... some are dangerously close to full-blown mental and physical breakdown.
"The US authorities should immediately stop pushing people to the edge with extreme isolation techniques and allow proper access for independent medical experts and human rights groups."
The provision that stripped detainees of their right to mount a legal challenge to their confinement was upheld by a US federal appeals court in Washington in February.
Pushing the anti-terror legislation through Congress last year, Mr Bush said he needed the new law to bring terror suspects to justice.
It allows for the indefinite detention of people as "enemy combatants".
The US has said it plans to use the military tribunal system to prosecute about 80 of 385 prisoners remaining at the camp.