Brazil's President Jair Bolsonaro supports heavy-handed tactics by police in their fight against organized crime, and has said gangsters should "die like cockroaches."
Rio de Janeiro's civil and military police regularly carry out deadly raids in the city's sprawling favelas.
At least 18 people died, one police officer, 16 alleged criminals and a female bystander, on Thursday when police raided the Alemao complex to take down an alleged criminal organization. The operation involved around 400 officers, four aircraft and 10 armoured vehicles.
"There are signs of major human rights violations, and the possibility of this being one of the operations with the highest number of deaths in Rio de Janeiro," the state public defender's office said in a statement.