Marines drive an amphibious assault vehicle through the Rocinha shantytown on Sunday.

Story highlights

NEW: A former police inspector says the operation will cut down on Rio's drug supply

Governor: "We are rescuing communities that were abandoned for decades"

Rio de Janeiro is trying to crack down on crime before the 2014 World Cup

Steep roads in Rocinha are covered in oil, possibly in an attempt to hinder police

Sao Paulo, Brazil CNN  — 

Rio de Janeiro’s special police forces declared Sunday that they were in full control of one of the city’s biggest and most notorious shantytowns, Rocinha, after a predawn operation aimed at wresting control from drug traffickers.

The operation, involving 3,000 police and security forces, had successfully occupied Rocinha and neighboring slums Vidigal and Chacara do Ceu, the Rio de Janeiro government said on its website.

“The next stage will be looking for the criminals that were able to get out of the places we are working,” military police Col. Alberto Pinheiro Neto told reporters Sunday morning.

About 100,000 people live in Rocinha.

Some 200 navy commandos with armored personnel carriers and helicopters also participated in the operation. Roads were blocked at 2:30 a.m. and troops started moving in around 4 a.m., according to the local government.

CNN affiliate Band News TV showed military assault vehicles rolling in and heavily armed police patrolling the streets.

The massive operation is part of Rio’s efforts to eliminate crime and arrest drug traffickers in one of the country¹s most violent cities ahead of the 2014 World Cup.

Rio de Janeiro Gov. Sergio Cabral said the so-called “Shock of Peace” operation was a “historic chapter.”

“We are rescuing communities that were abandoned for decades and dominated by parallel powers,” he told reporters. “These are people who need to be able to raise their children in peace.”

Residents told Band News TV that they were happy that police had stepped up their presence.

The operation will have an impact beyond Rocinha, which supplies 80% of the drugs in Rio de Janeiro, Congresswoman Marina Magessi told Band News TV.

“We are certain that from now until the end of the year, there will be a shortage of drugs present in Rio,” said Magessi, a former police inspector responsible for catching some of Rio’s top criminals.

Police have already “pacified” dozens of favelas since they began operations in 2008, but it’s an uphill battle. About one-fifth of Rio¹s residents live in the city’s 1,000 shantytowns, many of them perched on steep hills overlooking beachside condominiums.

Sunday’s operation stands in stark contrast to the invasion of the Alemao favela last year when more than 30 people were killed in shootouts.

Police arrested Rocinha’s top suspected drug trafficker days before they moved in. They found Antonio Francisco Bomfim, known as Nem, in the trunk of a car.

On Sunday, police reported capturing a handful of automatic guns and other weapons. They also found the steep and winding roads leading into Rocinha covered with oil, apparently an attempt to make it more difficult for police to enter.

CNN’s Marilia Brocchetto contributed to this report.