At least 64 individuals died in a law enforcement action cracking down on the Red Command gang in Rio de Janeiro's northern suburbs - the biggest and most lethal operation in the region's records.
Four police officers were among those killed during the security mission in the Alemão and Penha districts, local authorities said. Over 80 people were taken into custody.
Violent conflicts broke out when around 2,500 law enforcement officers initiated the operation on the morning of the operation. The Red Command responded with shooting, igniting obstacles and launching explosives via unmanned aircraft, police representatives said.
The International human rights organization said it was "horrified" by the law enforcement raid, demanding "immediate and thorough inquiries".
The slum neighborhoods adjacent to the city's international airport is regarded as the central command of among the nation's strongest organised crime groups.
State officials said a minimum of 50 of those killed were "classified by law enforcement as potential lawbreakers".
Dozens of people were wounded, including non-combatants affected by the violence.
Residents have compared the hillside area - home to about 300,000 residents - to a "war zone" since the mission commenced.
Access ways continue shut in the entire neighborhood.
The Red Command group have been expanding their territory across the Rio state, expanding narcotics distribution networks and control of the hillside favelas.
Local law enforcement reported over 200kg (441lb) of narcotics were captured in the operation, along with dozens of firearms.
The joint operation by different law enforcement agencies was preceded by a year-long investigation.
Numerous alleged criminals are believed to be participants in an organization from the northern region who are currently hiding in Rio de Janeiro.
Large-scale police operations are not uncommon in Brazil ahead of major international events in the country.
The United Nations climate conference the environmental summit is scheduled in the northern urban center in late autumn.
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