Seven people, including a boy of 14, have been killed in continuing violence between army-backed police units and Rio de Janeiro's criminal gangs, according to reports.
In the latest crackdown, soldiers and marines in camouflage and carrying rifles could be seen searching people as they came in and out of two poor neighbourhoods, known as favelas, within the posh Copacabana beach area.
The military, which was put in charge of security in Rio de Janeiro earlier this year, said the operation involved 1,800 troops and 50 police officers.
Their goal was to remove roadblocks put up by gangs and to "check up on reports of criminal activity and other illicit conduct," it said in a statement yesterday.
The operation caused large rush-hour traffic jams in the busy seaside area, but there was no sign of shooting incidents. However, similar raids frequently result in high body counts, including among unarmed bystanders.
On Wednesday, in a separate operation conducted in the sprawling network of favelas known as the Mare, police backed by armoured vehicles clashed with drug traffickers.
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Six people carrying weapons, ammunition and drugs were killed, according to the G1 news site.
A 14-year-old schoolboy who was hit by a stray bullet died of his wound later that night.
"The boy was late and was going to school when he was shot," municipal schools official Fatima Barros said on Globo television.
Some media reports said that the boy was fired upon from a helicopter that had been flying low over the favela, but late Thursday the teen's mother said her son had told her he was shot from an armoured vehicle.
The mother, Bruna Silva, was quoted by the state-run Agencia Brasil saying that her son might have survived if police had allowed an ambulance to reach him earlier.
"The ambulance took an hour. It couldn't enter. The police told it to turn around," she was quoted as saying. "My son was lucid, he was well. But when the ambulance arrived he was already red, going pale and cold."