The death toll in a major police raid targeting a drug trafficking gang in Rio de Janeiro has risen to at least 121 people dead.

The police operation was one of the most violent in Brazil's recent history, with human rights organizations calling for investigations into the deaths.

The deadliest police operation against drug gangs in Brazil's history killed at least 121 people, authorities said, as Rio de Janeiro residents lined a street with dozens of corpses found overnight.

The authorities said the deadliest operation in Rio’s history had killed four police officers and at least 60 other people. It was an attack on “narco-terrorists,” the state governor said.

The extraordinarily bloody episode shocked Rio, a city that is no stranger to scenes of gangs battling the police and each other. It also rattled Brazil just as the Latin American nation prepares to welcome delegations from around the world for the UN climate change conference, with events planned in Rio and the Amazonian city of Belem.

Rio state governor put death toll at around 60 on Wednesday, but warned that the real figure was likely higher.

The public defender's office in Rio de Janeiro said 132 people were killed in Brazil's deadliest police raid, residents lining streets with bodies.