Fighting broke out between two gang members at the Altamira prison in the northern Brazilian state of Para, Brazilian media reported.
The fighting then grew into rebellion. Part of the prison was set on fire in the riot, which began around 07: 00 in the morning.
At least 52 prisoners were killed in the fight, while authorities said 16 victims were decapitated and others were asphyxiated.
It took five hours for prison officials to quell the riot.
The fight broke out early Monday after members of one gang infiltrated another part of the jail. Video and pictures aired by local media showed flames shooting out of the prison.
Two prison officials were taken hostage but were reportedly freed unharmed.
ILKHA