A minimum of 64 people lost their lives in a police operation against the Red Command gang in the Brazilian city's north zone - the biggest and most lethal operation in the Brazilian state's records.
Four police officers were included in the fatalities during the police action in the Alemão and Penha districts, local authorities said. More than 80 individuals were taken into custody.
Violent conflicts broke out when approximately 2,500 security personnel moved in on Tuesday morning. The criminal organization countered with gunfire, setting fire to barriers and dropping bombs from drones, officials said.
The United Nations human rights body said it was "horrified" by the security action, calling for "prompt and effective investigations".
The slum neighborhoods near the aerial transportation hub is regarded as the headquarters of one of Brazil's most powerful illegal networks.
Local authorities stated no less than 50 of the fatalities were "identified by authorities as alleged offenders".
Many residents were hurt, including civilians caught in the crossfire.
Residents have likened the elevated neighborhood - inhabited by approximately 300,000 people - to a "war zone" since the mission commenced.
Access ways continue shut throughout the region.
The Comando Vermelho organization have been expanding their influence in the area, increasing drug trafficking routes and control of the hillside favelas.
Security forces said in excess of 200kg (441lb) of illegal substances were captured in the mission, together with dozens of firearms.
The combined action by civil and military police comes after a extended probe.
Dozens of the suspects are considered participants in an organization from the northern region who are at the moment taking refuge in the metropolitan area.
Major law enforcement actions are frequent in the South American nation before major international events in the country.
The UN climate summit the climate conference is scheduled in the northern city of Belem in late autumn.
