Rio de Janeiro (AFP) - Rioting inmates beheaded their rivals and bodies were burned in an explosion of violence in two Brazilian jails that left at least 18 people dead on Monday, authorities said.
Prisoners also took women visitors hostage, a regional official told Brazilian television.
It was the latest eruption of gruesome violence to hit the country's underfunded and overcrowded prison system.
The clash between two rival factions in a prison in the far northern state of Roraima killed 10 on Sunday, regional government media spokeswoman Jessica Laurie told AFP.
"The inmates were armed with stones and pieces of wood that they ripped from the walls. They used those bits of wood to decapitate their rivals. It was very brutal," she said.
"At first, the police thought there were more dead," she said. "Seven bodies were found burned and three others beheaded. Three inmates were lightly wounded and were treated."
On Monday a fresh riot, thought to be linked to the Roraima carnage, broke out at a jail in the northwestern state of Rondonia on the Bolivian border, a local police spokesman said.
"There are thought to be eight people dead but the bodies are burnt and the medical authorities will have to confirm" the toll, he told AFP.
Human rights groups have long complained about the deplorable conditions in Brazilian prisons, where fights and riots are frequent.
- Knives, clubs, hostages -
In the first riot in the Roraima state capital Boa Vista, the bloodshed began when inmates of one wing of the Agricola de Monte Cristo prison broke into another wing.
Prisoners were armed with knives and wooden clubs, an inmate's wife who was in the prison when the riot broke out told news channel G1.
Roraima state Secretary of Justice Uziel Castro said that the fight erupted during visiting hours, and some 100 relatives of inmates were briefly held hostage.
The rioters demanded that a judge come to hear their demands. Instead, special operations police stormed the prison, freed the hostages and regained control of the site by sundown.
"All the hostages were released," Castro said, adding that most of them were women.
The prison, some 3,400 kilometers (2,100 miles) northwest of Rio de Janeiro, is in a state that borders Venezuela and Guyana.
- Violent, overcrowded prisons -
Joana Moura, head of the union of Roraima penal workers, told the Folha de Boa Vista newspaper that the incident "is a reflection of the lack of interest from the state government" towards the prison system.
According to Moura, "there is no security equipment, there are not enough personnel for the tasks, and the prison officers are working beyond their capacity."
As of the end of 2014, there were 622,000 people imprisoned in Brazil, according to a Ministry of Justice report, which added that most of the prisoners are black males.
Brazil has the world's fourth largest prison population, the report said, after the United States, China and Russia.
Human rights groups have long complained about the deplorable conditions in Brazilian prisons.
In late May, 14 inmates died in prisons in the northeastern state of Ceara. That led the Inter-American Commission on Human Rights to issue a statement urging structural reforms.
In a separate incident in September, some 200 inmates rioted and escaped the overpopulated Jardinopolis prison in Sao Paulo state.