Elite police units backed by armored military vehicles and helicopters invaded Rio's largest slum before dawn Sunday, the most ambitious operation yet in an offensive that seeks to bring security to a seaside city long known for violence.

The action is part of a policing campaign to drive heavily armed drug gangs out of the city's slums, where the traffickers have ruled for decades.

Authorities vow to continue the crackdown and stabilize Rio's security before it hosts the 2014 World Cup and the 2016 Olympics. Officials are counting on those events to signal Brazil's arrival as a global economic, political and cultural power.

"Rocinha is one of the most strategically important points for police to control in Rio de Janeiro," said Paulo Storani, a security consultant and former captain in the elite BOPE police unit leading the invasion. "The pacification of Rocinha means that authorities have closed a security loop around the areas that will host most of the Olympic and World Cup activities."

The Rocinha slum is home to about 100,000 people living in flimsy shacks that sprawl over a mountainside separating some of Rio's richest neighborhoods. The location has made it one of the most lucrative and largest drug distribution points in the city.

Some estimates say the Friends of Friends gang that has controlled Rocinha and the neighboring Vidigal slum make more than $50 million in drug sales annually. Much of the sales are to tourists staying in the posh beach neighborhoods of Leblon, Ipanema and Copacabana and to middle and upper class Brazilians who live in them.

"This action is a huge blow to the structure of drug trafficking in Rio de Janeiro and against the second-largest drug faction," Storani said. "Beyond that, it's essential to have security in this area simply because of the huge number of people who circulate there."

The invasion of Rocinha comes near the end of a watershed year in the fight against drug gangs that rule more than half of Rio's 1,000 slums, where about one-third of the city's 6 million residents live.

Rio's program of installing permanent "police pacification units" in slums started in 2008.

The slums initially targeted were not among the most violent. But last November, gangs struck back with a weeklong spree of attacks that included burning buses and staging armed robberies of motorists on highways, spreading fear and chaos. At least 36 people died in the violence, mostly suspected drug traffickers fighting with police.

The surge of violence prodded police to invade with little planning the much-feared Alemao complex of slums on Rio's north side, near a highway leading to the international airport. Police routed the gangsters and took control within hours, imbuing the city with a new confidence that its security woes might be overcome even though most gang leaders had escaped capture.

A year later, the operation in Rocinha comes after careful planning and at a time chosen by authorities.

Police officials openly announced when they planned to invade Rocinha. They've used that tactic before and say it's led to less firefights during the incursions, with gang members either fleeing or simply laying down their weapons before police step foot in the slums. Up to 2,000 officers are expected to be involved.

In recent days, police set up roadblocks at Rocinha's entrances in a bid to capture the slum's drug kingpins.

The effort paid off Thursday, when police captured Antonio Bonfim Lopes, known as "Nem," who was the most-wanted drug trafficker in Rio. He was found hiding in the trunk of a car as it tried to flee the slum. His top lieutenants were also captured in recent days, leading many to think that police might take control of Rocinha with little fight.

In previous slum invasions, "we've not even needed to fire a single shot," said Jose Mariano Beltrame, head of Rio state security department and architect of the policing program. "But there is one variable we can't count on, and that's the intention of the criminal, of the gangster who is there inside."