Rio de Janeiro's state government says police have begun replacing the soldiers stationed as a "pacifying force" in part of the sprawling Mare slum.

The transition began early Friday in four of the 16 shantytowns that make up the Mare complex, which is near Rio's international airport and sandwiched between three key expressways.

Army soldiers moved into Mare about a year ago under a pacification program meant to push out drug trafficking gangs that ruled the slums for decades.

The program is seen as key to bringing security to violent Rio before the city hosts the 2016 Olympics, but it has recently come under criticism.

Residents of some "pacified" slums have accused police of brutality, and several highly publicized cases of officers killing unarmed residents have emerged.