At least 44 people have been killed, including nine in Rio de Janeiro on Wednesday, in a weeklong police operation against criminal gangs across Brazil.

The Rio raid came after days of deadly police crackdowns on drug trafficking gangs that have left 16 dead in Sao Paulo state and 19 killed in the northeastern state of Bahia.

Rio state police said officers had returned fire after coming under attack during a raid on a meeting of organized crime bosses in the Complexo da Penha group of favelas, on the city's north side.

Authorities face mounting calls for independent investigations of alleged police abuses in Brazil, where the security forces have been accused of human rights violations in their war with heavily armed drug gangs.

Police said the Rio operation came after officers received intelligence on a high-level meeting by gang leaders.

"A clash occurred when police teams came under attack by gunmen at the scene," state police said in a statement.

"Eleven suspects were wounded" and taken to the hospital, it said. "Nine of them died of their injuries."

Two policemen were also wounded and are in stable condition, it added.

Residents described the favela complex as a scene from a war zone during the raid, with locals left cowering inside their homes – mostly small shacks packed tightly on the hillsides.

"Businesses are all closed. People can't leave home to take their kids to school. You just have to take cover in a safe place and wait for the shooting to end," one resident told TV Globo, speaking on condition of anonymity.

AFP reporters outside the hospital where the wounded were taken described anxious residents waiting for news on injured relatives, flanked by a heavy police contingent as helicopters hovered overhead.

Police said makeshift barricades had been set up across the neighborhood to slow officers' advance.

They also said they had seized seven assault rifles, grenades and ammunition in the operation.

The dead included two gang leaders, police said.