Investigators suspect powerful crime groups, First Capital Command, of involvement in the attacks, local media say, in which gangs have torched vehicles and left police, businesses and government buildings riddled with bullets.
Brazilian security forces have waged a crackdown on criminal gangs accused of torching vehicles, shooting up buildings and sowing terror in four nights of attacks on dozens of northeastern cities.
One suspect was killed and 17 arrested in the crackdown on the attacks on Friday, which authorities say have been organised by jailed gang leaders as a protest against prison conditions and new restrictions on their activities from behind bars.
The security operation targeted gang members in three cities, including Natal, the capital of Rio Grande do Norte state, which has been at the centre of the unrest.
According to local media reports, more than 40 cities have been hit by the wave of violence, in which attackers have torched cars, trucks, buses and a health centre and left police cruisers, businesses and government buildings riddled with bullets.
President Luiz Inacio Lula da Silva's government has sent hundreds of federal officers to bolster state security forces.
The number of reinforcements will reach 500 by Saturday, said Rio Grande do Norte Governor Fatima Bezerra.
"We will win this fight, and we will get back to normal," she wrote on Twitter.
READ MORE: Explained: Why Brazil cities are facing wave of gang violence
Estamos trabalhando incansavelmente para restabelecer a ordem e trazer de volta a paz e a tranquilidade do povo do Rio Grande do Norte.— Fátima Bezerra (@fatimabezerra) March 18, 2023
Somando forças com apoio irrestrito do Governo Federal, e também dos estados da Paraíba, Ceará e do Pará.
Nós venceremos! Sigamos! https://t.co/Z6tY60SOhd
In all, three suspects have been killed in clashes with police and 97 have been arrested since the violence erupted on Tuesday.
According to media reports, investigators suspect one of Brazil's most powerful organised crime groups, the First Capital Command, of involvement in the attacks.
Brazil's severely overcrowded prison system is notorious for security failures and corruption, allowing crime bosses to continue running their operations behind bars.
The prison system has a history of deadly riots and clashes between rival gangs.
Rio Grande do Norte's prisons have been hit by allegations of severe abuses, including inmates being tortured and served rotting food.
READ MORE: Gang violence continues as Brazil sends more troops to northeast