A number of inmates at a jail in the northern Brazilian city of Belem died during an attempted mass breakout.
They were aided by an armed group outside the jail which used explosives against one of the prison walls.
One guard died, as well as 19 prisoners and those helping them from outside.
The fighting at the Santa Izabel Prison Complex outside Para's state capital was as intense as in warfare, the state security service said in a statement.
Four other guards were injured, one of them in serious condition.
The security service said the prison guards took on attackers from both inside and outside the prison.
The authorities are now trying to determine if any of the prisoners managed to escape.
Last year 56 people were killed in an uprising in a prison in the city of Manaus in Brazil's Amazon region.
Source: News Agencies, Edited by website team