Brazil prisoners beheaded in riot at Natal prison
At least 10 prisoners have been killed in fighting between rival gangs at the largest prison in the north-eastern Brazilian city of Natal, police say.
It took 14 hours for police to regain control of Alcacuz jail.
"The operation is still ongoing but the situation is now calm", a spokesperson told BBC Brasil.
This is the third major riot in Brazil this year. Nearly 100 inmates died in riots earlier this month in prisons in the states of Amazonas and Roraima.
The violence inside Alcacuz jail, in Rio Grande Do Norte state, began on Saturday afternoon, when members of one of the powerful criminal organisations present in most Brazilian jails attacked rivals in a separate wing.
"We could see the heads ripped off'' three of the dead prisoners, said Zemilton Silva, coordinator of the prison system.
Security forces managed to enter the outer area of the prison but decided to wait for daylight hours before going in on Sunday morning.
No prisoners were reported to have escaped. Police say the number of dead could go up.
Riots are not uncommon in Brazil's overcrowded jails, which are largely controlled by powerful criminal gangs.
But the scale of the violence this year has put pressure on President Michel Temer to address the problem.
After the Amazonas prison riot, the government announced plans to build five more high security jails and to create new intelligence units to try to curb the power of the gangs behind bars.