No less than 64 people lost their lives in a law enforcement action against the Red Command gang in the Brazilian city's northern suburbs - the largest and deadliest raid in the area's past.
Four security agents were among those killed during the security mission in the neighborhoods of Alemão and Penha, government representatives said. Over 80 people were taken into custody.
Fierce clashes broke out when about 2,500 law enforcement officers initiated the operation on the morning of the operation. The Red Command countered with gunfire, igniting obstacles and dropping bombs from drones, authorities stated.
The International human rights organization said it was "shocked" by the law enforcement raid, calling for "prompt and effective investigations".
The slum neighborhoods close to the city's international airport is seen as the main base of one of Brazil's most powerful illegal networks.
State officials stated no less than 50 of the fatalities were "classified by law enforcement as potential lawbreakers".
Many residents were hurt, including non-combatants affected by the violence.
Locals have compared the elevated neighborhood - inhabited by approximately 300,000 residents - to a "battlefield" since the operation began.
Roads remain closed in the entire neighborhood.
The Red Command group has been increasing their influence in the area, increasing drug trafficking routes and power in the informal settlements.
Security forces said more than 200kg (441lb) of narcotics were captured in the raid, in addition to numerous weapons.
The coordinated mission by various security forces was preceded by a extended probe.
Numerous alleged criminals are believed to be participants in an organization from the northern region who are at the moment taking refuge in Rio de Janeiro.
Large-scale police operations are regular in the country before major international events in the nation.
The United Nations climate conference the environmental summit is scheduled in the city of Belem in the north in November.
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