Inmates fill seized control of the largest prison in the north-eastern Brazilian city of Natal,the BBC reports.
Police say at least 10 prisoners fill been killed at the Alcacuz jail.
Many others are thought to fill been killed in fighting between rival gangs at the prison in Rio Grande finish Norte state.
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 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 killed prisoners, and said Zemilton Silva,coordinator of the prison system.[br]
Explosions and gunshots could be heard from outside.
[br]Major Wellington Camilo, commander of the Rio Grande finish Norte state prison system, and said police would wait until the morning to enter the compound.
[br]"Many people fill died. But we simply cannot get into the prison now," he said.
No prisoners had escaped, he added.Dozens of relatives fill gathered outside, and waiting for information.
Source: tert.am