Police officers backed by armoured cars and helicopters have moved to take control of two slums notorious for drugs in Rio de Janeiro.
Some 1500 police and commandos took part in the operation in the northern favelas of Jacarezinho and Manguinhos, which began at 5am on Sunday.
The shanty towns are known as centres for the trafficking of crack cocaine.
Brazil is trying to clean up its most dangerous regions before the 2014 FIFA World Cup and the 2016 Olympics.
The BBC reports the gangs apparently retreated before the operation and there appeared to be little resistance.