The New York-based Centre for Constitutional Rights said the hunger strike began because the US Department of Defence reneged on promises to negotiate with prisoners after a June-July hunger strike that involved up to 200 of the 500-plus men detained at the remote base.
The military said only 52 prisoners were involved in the first strike, after a released prisoner and lawyers who spoke to detained clients published the news.
"The DOD is practising dangerous deception," an attorney with the centre, Gitanjali Gutierrez, said in a statement.
"It hid evidence of the hunger strike and prisoner abuse from visiting senators and the public."
Spokesmen for the detention mission at Guantanamo could not immediately be reached for comment.
Detainees want to be charged and brought to trial, or freed. Many have been held for more than three and a half years without charge or access to lawyers. Most were captured in the Afghanistan war, suspected of ties to al-Qaida or the ousted Taliban regime that sheltered the terrorist network.
Defence officials promised to negotiate with prisoners when "the strike became so widespread that medics could not manage the need and elected to stop making their regular medical calls", and strikers spent 26 days without food, the statement from the New York-based centre said.
It quoted imprisoned British resident Shaker Aamer as telling a lawyer that when some of the men refused even water, "the conditions of some prisoners became so critical that the detainee hospital had no more beds and they had to take prisoners into the Navy hospital".
Mahdis Keshavarz, a spokesman for the centre, said she believed some 200 detainees were currently refusing all meals.
"Prisoners are now prepared to die in an effort to receive a fair hearing and humane treatment," Gutierrez said. "These men deserve their day in court."
Lawyers said they did know when exactly the new strike started. The lawyers said that as soon as the first strike ended, on 28 July, mistreatment of prisoners resumed and several prisoners allegedly were beaten by military personnel early in August.
A Kuwaiti prisoner was "violently assaulted" by the military's Extreme Reaction Force and subjected to psychological abuse, the statement said.