At least 64 civilians died in a police operation targeting the Red Command gang in the Brazilian city's north zone - the most extensive and fatal action in the Brazilian state's records.
Four security agents were included in the fatalities during the police action in the communities of Alemão and Penha, local authorities reported. Over 80 individuals were arrested.
Intense confrontations broke out when approximately 2,500 law enforcement officers moved in on Tuesday morning. The Red Command responded with gunfire, setting fire to barriers and launching explosives via unmanned aircraft, authorities stated.
The International human rights organization stated it felt "horrified" by the law enforcement raid, demanding "prompt and effective investigations".
The informal settlements adjacent to the city's international airport is regarded as the headquarters of among the nation's strongest criminal organizations.
State officials confirmed at least 50 of the deceased were "identified by authorities as alleged offenders".
Many residents were injured, including civilians caught in the crossfire.
Residents have described the mountainous region - inhabited by approximately 300,000 individuals - to a "combat area" since the mission commenced.
Roads remain closed throughout the region.
The Red Command group has been increasing their influence in the area, expanding narcotics distribution networks and power in the informal settlements.
Security forces said over 200kg (441lb) of narcotics were confiscated in the raid, along with dozens of firearms.
The coordinated mission by various security forces follows a extended probe.
Many of the individuals are believed to be members of a faction from the northern region who are presently concealed in Rio de Janeiro.
Major law enforcement actions are not uncommon in the South American nation before important worldwide conferences in the country.
The United Nations climate conference the environmental summit is taking place in the northern city of Belem in the eleventh month.
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