The Moroccan Human Rights Association (AMDH) said on Monday that 29 inmates of three prisons, one of them in the disputed territory and two in northern Moroccan cities, had refused to eat for three weeks.
"The strike has started to seriously take its toll on their health. Their lives are at risk now," said Abdelilah Benabdeslam, a spokesman for the association.
A Justice Ministry official said only 20 to 22 detainees were on hunger strike.
They want to be moved to jails that are closer to their relatives for visits and the lifting of Morocco's heavy security deployment in Western Sahara's capital, Laayoune, said the AMDH, which has visited some of the detainees.
A total of 37 residents were detained during and after anti-Moroccan riots in Western Sahara in May.
A dozen of the 37 have been handed jail terms of up to five years for offences including sabotage of public property and use of weapons against public officials.
The rest are to be tried next month.
"The verdicts were the results of unfair trials and those awaiting to be tried have been held for much longer periods than what the law stipulates, in clear violation of their basic rights as defendants," Benabdeslam said.
"The strike has started to seriously take its toll on their health. Their lives are at risk now"
Abdelilah Benabdeslam, Moroccan Human Rights Association spokesman
Human rights groups say some of the detained have been tortured, a charge denied by Moroccan authorities.
The AMDH and two other rights groups urged authorities to hold talks with the detainees to try to end the hunger strike.
Claiming centuries-old historic rights over the desert territory, Morocco reclaimed Western Sahara after colonial power Spain pulled out in 1975.
Authorities say the May riots were instigated by supporters of the Polisario Front, which seeks independence for the territory. Several people were hurt in clashes with police.
The Polisario Front, based in Algeria, urged the African Union this past weekend to intervene and help secure the release of the 37 detainees, whom it called "political prisoners".