At least 52 inmates were killed in a prison riot in northern Brazil on Monday as rival gang factions fought each other, an official said.
Sixteen of the dead were decapitated in the second major eruption of violence to rock the country's severely overpopulated and deadly prison system in as many months.
Fighting broke out in the Altamira Regional Recovery Center at around 7:00 am (1000 GMT), an official from the Para state government's penitentiary department told AFP.
Two guards were taken hostage during the hours-long clashes, which were brought under control at around midday. They were eventually freed.
Brazilian TV stations showed footage of thick black smoke rising from the prison compound and people sitting on the roof of a building.
Other images showed flames inside a building that almost reached the ceiling and people, apparently prisoners, sitting on the ground outside.
"It is likely that many detainees died from asphyxiation (from smoke)," the government official said, adding the death toll could rise.
Around 300 prisoners were being held at the jail, the official said, which reportedly has a capacity for 200.
An outbreak of violence in the same prison in September 2018 left at least seven inmates dead, local media reported previously. Guards had apparently foiled an attempted prison escape.
In May this year, at least 55 prisoners were killed in several jails in the neighboring state of Amazonas in violence also blamed on an apparent drug trafficking gang dispute.
Most of the victims were killed by asphyxiation, the state government said at the time.
The federal government dispatched reinforcements to boost security in the jails.
Brazil has the world's third largest prison population after the United States and China, with 726,712 inmates as of June 2016, according to official statistics.
The population is roughly double the capacity of the nation's jails, which in the same year was estimated to be 368,049 inmates.
The federal government had been expected to add another 115,000 inmates by the end of 2018, Human Rights Watch said in a recent report.
Around 40 percent of the prison population is made up of pretrial detainees.
Along with severe overcrowding and gang violence, riots and breakout attempts in Brazil's prisons are not uncommon.
Experts have described conditions in the country's jails as inhuman, with most inmates poor, black and with little formal education.