By the end of the weekend, 300 soldiers should be patrolling that city and other towns in Ceara state in a bid to halt the rampage, national public security secretary Guilherme Teophilo said, according to government news agency Agencia Brasil.

The intervention is the first test of new President Jair Bolsonaro's strict law-and-order platform since he took office last Tuesday.

His justice minister ordered the deployment after evaluating that Ceara police were overwhelmed. Some 50 suspects have been arrested.

The gangs terrorizing Fortaleza could be seen torching service stations in security videos aired by Brazilian media. Dozens of attacks have been registered this week, forcing residents to stay at home and leaving main roads deserted.

The trigger of the wave of violence was being investigated, but intelligence reports published by media suggested gangs were revolting against tough new measures recently imposed in the state's prisons.

The changes include cellphone signal blocker and an end to a policy of separating inmates according to gang affiliation.

Two gangs have set aside their rivalry to join forces against the government, the G1 news website reported, citing security officials.

Bolsonaro has vowed to crack down on Brazil's rampant crime by extending immunity to soldiers and police using lethal force and easing gun laws so "good" citizens can challenge armed criminals.

The far-right president, a 63-year-old former paratrooper, has made "restoring order" a centerpiece of his four-year mandate.

Much of that task falls to his justice minister, Sergio Moro, a former star judge who headed up Operation Car Wash, an investigation into Brazil's biggest-ever corruption scandal.