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Brazilian forces backed by armored vehicles and helicopters swept into Rio de Janeiro’s largest slum on Sunday, part of an effort to drive out heavily armed drug gangs and improve security in the seaside city.

The operation in Rocinha, a hillside shantytown overlooking some of Rio’s wealthiest neighborhoods, began before dawn, according to news reports. Police and soldiers, trailed by reporters, climbed up winding roads as helicopters roared overhead.

Within hours, authorities said they had taken control of the area without firing a shot. Officials had announced their plans days in advance, giving drug gangs enough time to clear out, Reuters news agency reported.

PHOTOS: Brazil cracks down on crime

Security forces also went into the nearby Vidigal slum. However, the Associated Press reported that armored personnel carriers were having trouble maneuvering there, apparently because drug gangs had covered the steep roads in oil.

Brazilian authorities have vowed to crack down on gang violence before the country hosts the soccer World Cup in 2014 and the Olympics two years later.

At least one Rocinha resident applauded the effort Sunday. “Tell the world we’re not all drug traffickers! We’re working people and now they’re coming to liberate us,” a man was quoted as saying by the Associated Press.

Others were less certain about the operation. “Lord knows if there will be war or peace, or even if things will be better if police take this slum,” Marisa Costa da Silva, who runs a small candy shop, told AP. “We’ve heard they’ve been abusive to slum residents in other places they’ve taken. I have no idea what to expect.”

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