The US military said that 76 men have refused nine consecutive meals over three days.
But lawyers for the inmates said at least 210 men have been on hunger strike for the past three weeks.
More than 500 inmates are currently being held at Guantanamo. Only four have been charged including Australian man David Hicks.
In a statement, the US military said the detainees have been given intravenous hydration, Gatorade, water and a nutritional supplement.
But the Pentagon’s version contrasted from the account of two Afghans released from the facility on Wednesday.
Former detainee Habir Russol said about 180 Afghan prisoners were not eating or drinking, adding some had been on a fast for up to 15 days.
The other freed detainee, Moheb Ullah Borekzai, told the Associated Press the detaineesaere protesting because “some of these people say they were mistreated during interrogation. Some say they are innocent.”
“They are protesting that they have been in jail nearly four years and they want to be released,” he said.
The two Afghans said they had been accused of being members of the former Taliban regime, but both said they were innocent.
Neither said how long they had been detained.
Human rights lawyers are also concerned at the length of time it is taking to bring the detainees before the military commissions.
“Since January 2002, the (US Defence Department) has denied prisoners access to the courts or legal counsel in an effort to avoid justifying the basis for the detentions, said lawyer Gitanjali Gutierrez.
Mr Gutierrez is associated with the New York-based Centre for Constitutional Rights, which is representing some of the prisoners.
“This policy has driven detainees to strike until they die or are afforded a fair hearing and humane treatment,” he added.
News of the hunger strike comes just 24 hours after US Defence Secretary Donald Rumsfeld announced changes to the military trials of Guantanamo Bay detainees.
The changes will make the proceedings similar to the judge and jury model.
Panels hearing the cases will be split between a presiding officer who will act as judge, and members who will decide the verdicts.
The procedures remove the presiding officer from voting on findings and sentencing.
These responsibilities will be taken up by the other panel members.
Until now, the three panel members have together determined findings, decided legal questions and imposed sentences.
The Australian government has welcomed the changes which will apply to Mr Hicks, one of the four detainees slated for trial.
But his American military lawyer, Major Michael Mori, has branded the changes as “meaningless.”
“This is totally cosmetic. It’s totally for show. It’s because they realise that no-one in the world accepts it as a fair system and are desperate to convince somebody,” said Major Mori.
“You can slap a new coat of paint on the outside of a house with broken foundations, but it doesn’t fix the problem,” he said.