At least 70 prisoners held in Black Beach prison in Equatorial Guinea face death by starvation, says human rights group Amnesty International.
The group includes dozens of political detainees arrested last year and held without charge or trial, it says.
There are also 11 foreigners held convicted of trying to overthrow the government in June 2002 in a trial the human rights group says was unfair.
Since the end of February, little food has been provided, the group says.
This leaves prisoners and detainees dependent on food brought by families.
Authorities had provided prisoners with a cup of rice a day until December, when the ration was cut to one or two bread rolls a day, Amnesty said.
Those without families have no access to food, it said.
"Many of those detained at Black Beach prison are already extremely weak because of the torture or ill-treatment they have suffered and because of chronic illnesses," Amnesty said.
"Unless immediate action is taken, many of those detained at Black Beach prison will die," said Kolawole Olaniyan, director of Amnesty International's Africa Programme.
Black Beach is in Malabo, the capital, which is located on a volcanic island in the Gulf of Guinea.
Equatorial Guinea says the allegations are untrue and accused Amnesty of seeking to tarnish its image.
"Prisoners in Equatorial Guinea are not going hungry. We have assured their basic rights," Second Deputy Prime Minister Ricardo Mangue Obama Nfube, whose portfolio includes human rights, told Reuters news agency.