At least nine prisoners have been killed in five days of unrest at the Urso Branco (White Bear) prison in Rondonia state, which ended on Thursday.
Hungry prisoners have been reduced to hunting for cats to eat, a police spokesman said.
No breakthrough has been made in negotiations between the authorities and uprising leaders, who took over the prison on Sunday in a protest against overcrowding.
Riots are common in Brazil's prisons but this one has been particularly brutal.
On Monday, rebel inmates threw the mutilated corpses of rivals from the walls around the prison. They decapitated two of the dead men and hung their heads from the wall.
The inmates also strung up three dead bodies by their feet as horrified relatives waiting outside watched.
The spokesman said five inmates had jumped from a seven-metre high wall and run to police for protection, fearing they would be killed. Local media said they told police at least three more corpses were inside the prison, which would raise the death toll to 12.
The inmates' main demand of replacing the prison director has been met but there was no sign of an end to the uprising.
Electricity and water at the prison have been turned off and food is short.
"They are hunting cats inside, but there are not too many of them around to feed on," the police spokesman said.
The prison was built for 360 inmates but houses more than 1000.
The Organisation of American States' human rights watchdog criticised the Brazilian government for not protecting the prisoners in the Amazon jail, which was the scene of a massacre in 2002 in which 27 inmates were killed in gang fighting.