At least 52 prisoners were killed by other inmates during clashes between organised crime groups at the Altamira prison in northern Brazil on Monday, according to prison officials.
Para state prison authorities said 16 of the victims were decapitated while others were asphyxiated.
The count of victims could rise when authorities have searched all areas involved, state prisons chief Jarbas Vasconcelos said at a news conference.
Vasconcelos said a fight between criminal groups at the Altamira prison erupted around in the morning, when breakfast was being served.
Inmates also set fires that prevented authorities from entering parts of the facility for about five hours, he said.
The killings echoed those of 55 inmates who died in a series of riots in May in several prisons in the neighbouring state of Amazonas.