Overcrowding is a factor behind the frequent riots in Brazil's jails

On Monday two prisoners were decapitated on the roof of the Urso Branco prison in Rondonia state. One body was torn to bits.

Some 170 prison visitors are also being held inside the jail. Some media have suggested they could be used as shields in the event of an assault by police.

Inmates in the seriously overcrowded jail have demanded better conditions.

Brawl

The rebellion appears to have started when two prisoners were killed in a brawl on Friday.

In the ensuing riot prisoners demanded better food and more regular meals, the Folha de Sao Paulo newspaper reported.

Their demands now reportedly include the loosening of prison visit rules and the release of prisoners who have served out their sentences but remain in detention.

As the rebellion spread over the weekend, prison gangs appeared to take advantage of the situation to carry out revenge killings.

An inmate was killed on Sunday, followed by five or six further deaths on Monday, reported by inmates to Brazilian media.

But officials have confirmed only six deaths.

In Monday's bloody scenes, two men were beheaded atop the prison roof. One man's body was cut to pieces, and thrown from the roof. The other man's head was held up by his attackers.

Some 170 friends and relatives of the inmates are also inside the prison. It is unclear whether they are hostages or are willing supporters of the uprising.

Folha de Sao Paulo suggests the visitors are being used as a shield against an assault by dozens of military police now surrounding the jail.

Obstacles to solution

State governor Ivo Cassol has cancelled a trip to deal with the crisis. The legal ombudsman, human rights representatives, military police, and public security officials are also among the negotiators.

But prison director Luis Pereira Rodrigues told AP news agency the prisoners had "totally unreasonable" demands, including the right to have prostitutes visit them inside and to take drugs.

A public security spokesman told AFP news agency negotiations were hampered by the inmates' lack of a clear leader to negotiate on their behalf.

The Urso Branco jail was built to house 350 inmates but currently holds at least 1,300.