Eleven people were shot dead in a slum on the north side of Rio de Janeiro early on Tuesday during a police raid to capture heads of a drug-trafficking organisation.
The deaths included a woman who was hit in the exchange of gunfire between gang members and police in the Vila Cruzeiro favela, as Rio slums are called.
Police said the other dead were suspected gang members who resisted the enforcement operation and opened fire on police with automatic weapons.
Residents said on social media that heavy shooting began in darkness at 4:00 am in a wooded area next to Vila Cruzeiro, causing fear and panic in the slum.
The police operation, backed by a helicopter, targeted leaders of Rio's largest organised crime gang, the Comando Vermelho, who were hiding in Vila Cruzeiro according to police intelligence.
"It was a very intense confrontation," Colonel Ivan Blaz, spokesman for the militarized police force that led the operation, told reporters.
He said the woman could have been hit by a shot fired from inside Vila Cruzeiro.
Police seized assault rifles, pistols and a grenade, as well as cars and motor bikes used by the criminals, he said.
The dead woman was identified as 41-year-old Gabriele Cunha, hit by a stray bullet even though she lived in a neighbouring slum called Chatuba.
Schools did not open for classes.
Last year, 28 people died in violence in Rio slums, mostly in clashes between drug traffickers and police.
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