The US has just missed a deadline to close the prison camp
The American Civil Liberties Union has criticised a recommendation that 47 Guantanamo Bay inmates should be held indefinitely without trial.
Justice department officials said the men were too dangerous to release, but could not be tried as evidence against them would not stand up in a US court.
ACLU executive director Anthony Romero said their detention would reduce the camp's closure to a "symbolic gesture".
The White House said the president did not have to accept the recommendation.
It came as the deadline President Barack Obama had set himself on his second day in office for closing the prison camp at Guantanamo Bay passed.
'Not evidence at all'
Earlier on Friday, officials said a task force led by the justice department had recommended that while 35 detainees could be prosecuted through trials or military tribunals, 110 could be released either now or at a later date.
Just as important as closing the prison quickly is closing it right
Anthony Romero American Civil Liberties Union
The other 47 detainees were considered too dangerous to release, but could not be tried because the evidence against them was too flimsy or was extracted from them by coercion, so would not hold up in court, it concluded.
In a statement, the ACLU said it disputed that any significant category of such detainees existed, and renewed its call for the closure of the prison.
"If there is credible evidence that these prisoners are dangerous, there is no reason why that evidence could not be introduced against them in criminal trials," said Jameel Jaffer, director of the ACLU National Security Project.
"The criminal laws, and the material support laws in particular, are broad enough to reach anyone who presents a serious threat, and the federal courts are fully capable of affording defendants fair trials while protecting the government's legitimate interest in protecting information that is properly classified."
Mr Jaffer said evidence that had been "tainted" according to the task force's recommendation, was "not evidence at all". The US justice system, he added, "excludes coerced evidence not only because coercion and torture are illegal, but because coerced evidence is unreliable".
"Just as important as closing the prison quickly is closing it right, and that means putting an end to the illegal policy of indefinite detention without charge or trial," said Mr Romero.
Former detainee Moazzam Begg criticised Mr Obama for failing to keep his promise on closing Guantanamo Bay.
"The whole talk that change has come to America is simply a lie," he told the BBC.
The BBC's Adam Brookes in Washington says the recommendation will dismay many of Mr Obama's supporters, who had hoped the president would end the practice of detention without trial.
However, a White House official stressed that this was only a recommendation, which Mr Obama did not have to accept. The task force's findings will also be subject to review by the National Security Council.
Congress has laid down that only those to be tried can be moved to US soil, so the question of what to do with those who officials want to be detained indefinitely without trial has yet to be resolved.
More than 40 detainees have been transferred out of the prison during Mr Obama's first year in office.
But diplomatic hurdles and domestic opposition to the government's plan to house suspects on US soil have hampered his plans to close it down completely.
Plans to move detainees approved for trial to a prison facility in Illinois remain under consideration.
The task force recommended that among those cleared for release, 80 detainees, including about 30 Yemenis, could be freed immediately, the Washington Post said.
The panel said the release of another 30 Yemenis should be contingent on an improved situation in Yemen, the newspaper reported.
However, the US recently suspended the repatriation of Yemeni prisoners indefinitely, following an airliner bomb plot that was allegedly planned in Yemen.
Yemenis account for approximately half of the inmates at Guantanamo.
Mr Obama set himself the 22 January closure deadline a year ago, shortly after being sworn in.
He has subsequently said he wants the camp closed this year, without setting a specific deadline.
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