At least 57 prisoners were killed by other inmates during clashes between organised crime groups at the Altamira prison in northern Brazil.
A fight between criminal groups at the Altamira prison erupted around in the morning, when breakfast was being served.
Members of the Comando Classe A (CCA) gang set fire to a cell where rival gang members from Comando Vermelho (Red Command) were kept, the Pará state government said in a statement.
Officials said that the two prison officers taken hostage were soon released, because the goal of the attack was to strike at the rival gang – rather than at the prison guards.
Para state prison authorities added,16 of the victims were decapitated while others suffocated after the prison buildings were set on fire.
Gang leaders involved in the attack were to be moved to federal prisons for isolation, if they were identified.
Violence in Brazilian prisons is not uncommon. The country has the world’s third-largest prison population of some 700,000 people, and overcrowding is a widespread problem.