At least 52 prisoners have been killed by other inmates during a battle at the Altamira prison in northern Brazil, AP has reported.
According to prison officials, 16 of the victims were decapitated in the five-hour-long clash while others were asphyxiated.
The fact that inmates also set a part of the jail on fire meant that prison authorities were unable to enter parts of the facility, AP says.
A video has emerged online, showing a part of the prison engulfed in flames and smoke.
The riot comes two months after 55 prisoners were killed in a battle in the northern Brazilian state of Amazonas.