Thirty-three inmates have died in a prison killing spree led by Brazil's largest gang, with some having their hearts and intestines ripped out, authorities said.
The bloodshed came days after 60 inmates were killed during rioting at two prisons in a neighbouring state, and increases fears that violence could spread, including to the streets of major cities, as gangs vie for influence and territory inside prisons and in slums where trafficking operations are often based.
It has also becoming a flashpoint for the government of President Michel Temer, whose administration is already struggling with an economic crisis and mounting corruption allegations.
"This is a national crisis," said Uziel Castro, security secretary of the state of Roraima, where the latest massacre happened.
Mr Castro said the killing spree at the Agricultural Penitentiary of Monte Cristo, in the town of Boa Vista, was led by members of Sao Paulo-based First Command, Brazil's biggest criminal organisation.
He said First Command members did not attack members of a rival gang but other prisoners, for motives that are not yet clear.
"There was no confrontation, this was a killing spree," said Mr Castro. "It was barbaric. Some were beheaded, others had their hearts or intestines ripped out."
He said firearms were not involved and none of the 1,500 inmates in the prison escaped.
It was not clear whether there was a connection to the rioting earlier this week in the neighbouring state of Amazonas, which officials blamed on a gang war between First Command and the Family of the North gang, which fight over control of prisons and drug routes in northern Brazil along the borders of Colombia, Venezuela, Peru and the Guianas.
A police statement said officers, including a heavily armed military-type riot squad, had been deployed to the prison.
As details about the latest disturbance were emerging, justice minister Alexandre de Moraes announced measures to curb the violence.
He said federal police would be more integrated in state capitals and special taskforces would be created to more quickly process criminal charges, a measure aimed at reducing overcrowding.
He offered no deadlines for the initiatives but said they would "be realistic" given the recession in Latin America's largest economy.
"The situation isn't out of control," said Mr de Moraes. "It's (just) another difficult situation."
The rioting on Sunday and Monday in Amazonas included the country's worst prison massacre since 1992, with half of the 56 killed at one institution beheaded and several others dismembered.
A total of 184 inmates escaped from Amazonas prisons in the disturbances. By Thursday afternoon, only 65 had been recaptured.
In October, a riot at the Agricultural Penitentiary of Monte Cristo left 10 dead. Authorities said that clash involved the First Command and Red Command, which has its base in Rio de Janeiro. On the same day, eight were killed in a prison in the state of Rondonia, which borders Bolivia.
After that clash, Mr Castro asked the federal government for help with the prison, according to a letter obtained by the Associated Press.
He requested that the national guard be sent in to help because the moving of prisoners deemed leaders of the rebellion had led to threats of retaliation.
Asked about the request, Mr de Moraes acknowledged that help had not been sent.
Mr Castro said: "Criminals are trying to kill each other. What we need is the federal government help. This is all over Brazil."