At least 64 civilians died in a law enforcement action targeting the Red Command gang in the Brazilian city's northern suburbs - the largest and deadliest raid in the region's records.
Four security agents were included in the fatalities during the security mission in the Alemão and Penha districts, government representatives reported. In excess of 80 individuals were detained.
Fierce clashes broke out when approximately 2,500 law enforcement officers initiated the operation on Tuesday morning. The Red Command countered with armed resistance, igniting obstacles and launching explosives via unmanned aircraft, officials confirmed.
The United Nations human rights body said it was "appalled" by the security action, calling for "immediate and thorough inquiries".
The informal settlements adjacent to the aerial transportation hub is considered to be the central command of among the nation's strongest organised crime groups.
Local authorities confirmed at least 50 of the fatalities were "identified by authorities as alleged offenders".
Many residents were hurt, including civilians caught in the crossfire.
Residents have compared the mountainous region - home to about 300,000 residents - to a "war zone" since the raid started.
Roads remain closed across the area.
The Red Command group has been growing their control throughout the region, expanding narcotics distribution networks and power in the informal settlements.
Security forces said in excess of 200kg (441lb) of drugs were confiscated in the raid, together with numerous weapons.
The coordinated mission by different law enforcement agencies follows a extended probe.
Dozens of the suspects are thought to be participants in an organization from the northern region who are at the moment taking refuge in the metropolitan area.
Large-scale police operations are frequent in Brazil ahead of major international events in the country.
The international environmental meeting the climate conference is set to occur in the city of Belem in the north in late autumn.
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