The brawl started on Sunday afternoon when members of the Familia do Norte (FDN), the gang that controls the prison, picked a fight with members of the First Capital Command (PCC), a criminal organisation based in Sao Paulo, EFE news reported.

Dozens of inmates were beheaded, mutilated and set on fire during the fight, which the FDN started to send a "message" to the PCC about its power in the state, Amazonas Public Safety Secretary Sergio Fontes said in a press conference.

"There were only deaths on one side. The FDN massacred the suspected members of the PCC and a few other inmates," Fontes said.

The FDN is an ally of the Red Command (CV), a gang from Rio de Janeiro that is a rival of the PCC.

The incident was the "biggest massacre in the prison system" in Amazonas and produced the highest loss of life ever at the hands of inmates in a penitentiary fight in Brazil, the Public Safety Secretariat said.

An uprising 24 years ago at Sao Paulo's Carandiru prison left 111 inmates dead, but the majority of the deaths were at the hands of police who stormed the penitentiary.

"Unfortunately, we are looking at the largest and most horrible massacre to occur in Brazil's prisons," the head of the Brazilian Bar Association (OAB) Human Rights Commission in Amazonas state, Epitacio Almeida, said.

The OAB assisted the inmates with the negotiations and worked to get the 12 hostages taken during the riot released.