The violence erupted at the Alcacuz correctional center, when members of one gang were able to storm into another where rival gang members were held.
The violence erupted at the Alcacuz correctional center, the biggest in the state of Rio Grande do Norte, when members of one gang were able to storm into another where rival gang members were held.
"There are at least three inmates dead because we were able to see their heads," state prisons manager Zemilton Silva told local media. Some local media said as many as 10 could have been killed.
The prison, just outside the state capital Natal, is built for a maximum of 620 but holds 1,100 inmates.
While real and problematic overcrowding is a fact, it is far from the only factor in the violence.
Many experts say it was part of a war between organized drug gangs in one of the world's most important cocaine markets and trafficking routes.
Last week's series of massacres left 100 prisoners dead -- many of them active members of gangs, the authorities said.
The government has deployed 200 emergency personnel to secure the two prisons where the most blood was shed.
The largest bloodbath appeared to be an orchestrated mass killing targeting members of Brazil's biggest gang, the First Capital Command (PCC).
It was thought to be a backlash by the PCC's rivals for its violent expansion.
With an estimated 20,000 members, the PCC thrives even with its leader Marcos Willians Herbas Camacho, known as "Marcola," behind bars since 1999.
Investigators say that apart from its drug-trafficking activities, it also owns bus companies, minor football clubs and an illicit gasoline refinery.
Its rival Red Command is considered Brazil's oldest gang, dating to the 1970s.
It thrived on a cocaine boom from the 1980s, expanding from bank robberies and kidnapping to control the drug trade in Rio.