A prison clash in an Altamira jail in the Brazilian state of Pará left at least 52 people dead, the state's prison authority said on Monday.
During a press conference, the official said 16 of the dead were decapitated.
The clash started with rival gang members fighting each other for five hours. Local media reported that gang members in one prison block invaded a gang in another block.
During the clash, gang members took at least two penitentiary officers hostage who have been released.
The violence started at around 7:00 local time (12pm CEST) and ended at around noon.
In January 2017, nearly 150 prisoners died during three weeks of violence in several Brazilian prisons as local gangs backed by Brazil's two largest drug factions attacked one another.
Brazil's president Jair Bolsonaro has said he wants to impose tighter controls in the country's prisons as well as build more of them.
Brazil's incarcerated population has increased eight-fold in three decades to around 750,000 inmates — the world's third-highest prison population.