At least 64 civilians died in a police operation cracking down on the Red Command gang in the Brazilian city's northern districts - the biggest and most lethal operation in the area's history.
Four police officers were included in the fatalities during Operation Containment in the neighborhoods of Alemão and Penha, government representatives reported. Over 80 people were taken into custody.
Fierce clashes broke out when about 2,500 law enforcement officers entered the area on the morning of the operation. The criminal organization reacted using shooting, setting fire to barriers and using drone-dropped explosives, police representatives stated.
The UN Human Rights Office stated it felt "horrified" by the police operation, calling for "immediate and thorough inquiries".
The informal settlements adjacent to Rio's airport is regarded as the central command of among the nation's strongest organised crime groups.
State officials said no less than 50 of those killed were "identified by authorities as alleged offenders".
Many residents were hurt, including non-combatants affected by the violence.
Residents have compared the elevated neighborhood - home to about 300,000 people - to a "war zone" since the operation began.
Access ways continue shut in the entire neighborhood.
The Red Command group has been growing their influence in the area, increasing drug trafficking routes and power in the informal settlements.
Local law enforcement said over 200kg (441lb) of drugs were confiscated in the raid, together with numerous weapons.
The combined action by various security forces follows a year-long investigation.
Dozens of the suspects are thought to be affiliates of a group from the Pará state who are at the moment taking refuge in Rio de Janeiro.
Major law enforcement actions are frequent in Brazil before important worldwide conferences in the South American nation.
The international environmental meeting COP30 is set to occur in the northern city of Belem in late autumn.
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