At least 20 people were killed when police in Rio de Janeiro raided the favela of Villa Cruzeiro, during which violent clashes broke out, official sources said.
The operation was aimed at locating and arresting criminal bosses, some of whom are from other countries, according to a police statement, AP reports.
It is added that a fire was opened on members of the police, after which one favela resident was shot and killed on the spot, and the local press announced that it was a 41-year-old woman hit by a stray bullet.
It is also stated that 11 wounded people were found after the shooting and transferred to a nearby state hospital.
As the day wore on, more firearms victims were brought to the same hospital, some of whom were already dead, according to the AP.
The ambulance service confirmed 12 deaths and five people undergoing treatment in the trauma department, the state health secretariat said.