Brazilian police backed by armoured military vehicles have invaded Rio de Janeiro's biggest slum.
Hundreds of black-clad, rifle-armed officers began swarming into the Rocinha shantytown early on Sunday.
Experts say it is the most important step yet in bringing security to Rio de Janeiro before it hosts matches in the 2014 World Cup and the 2016 Olympic Games.
About 100,000 people live in Rocinha, which is the biggest drug distribution point in Rio. The slum straddles a green mountainside above upmarket neighbourhoods.
The action is part of a policing programme that sees elite police units force heavily armed drug gangs from shantytowns they have controlled for decades, before authorities set up a permanent police presence.
Officials are counting on the major sporting 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 raid.
"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 neighbourhoods. 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 neighbouring Vidigal slum make more than 50 million dollars in drug sales annually.
Much of the sales are to tourists staying in the posh beach neighbourhoods 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," Mr 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 raid 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 six 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 prompted police to invade 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.