(CNN) -- Authorities launched a massive sweep of the Alemao favela complex in northern Rio de Janeiro, Brazil, Sunday morning.
Military police told reporters they were entering the virtually impenetrable labyrinth of slums, searching for drug gang leaders and illegal drugs.
CNN affiliate TV Record showed images of armored vehicles entering the slum Sunday morning, followed closely by police trucks and helicopters equipped with military rifles.
Police told TV Record that they had arrested at least nine people, confiscated heavy weapons and seized four tons of marijuana.
On Saturday, police spokesman Mario Sergio Duarte declared an ultimatum against drug gangs in the complex, stating that a military attack was imminent.
"We will not retreat in our decision to bring peace to Rio," Duarte said. "We are in our final stages of reaching the traffickers in the Alemao complex."
Police had offered drug gang leaders the chance to surrender, but they refused, Brazilian military police Col. Lima Castro said in an interview with CNN affiliate Band News Sunday.
Castro said police were asking any residents who live in the slum to stay home and take cover to avoid being hit by any stray bullets during the takeover.
A police officer was hit in the arm by a bullet as authorities entered, police told TV Record.
The Red Cross sent 50 volunteers to help with the injured and there are four vehicles throughout the complex capable of helping with basic nursing and first-aid needs, the agency told Band News.
Police said that some criminals were threatening taxi drivers to get them out of the slums, and some neighboring favelas said that the drug traffickers simply migrated to their neighborhoods, both television stations reported. There were also reports of suspects trying to escape through sewer lines running underneath the slum.
Some 2,600 police and soldiers occupied the favela Sunday, and Castro said that there is no target time to leave.
Army Gen. Adriano Pereira Jr. said they will hold their positions as needed.
"We are serving a request by the department of defense with authorization by President Lula. We will stay for as long as its necessary," he said.
At least 35 people have died, 174 arrested and 123 detained since violence broke out last Sunday as a response by drug gangs to an increased police presence in the crime-ridden slums, known as favelas, police said, according to the official Agencia Brasil. More than 96 vehicles have been burned.
Brazil's Defense Minister Nelson Jobim has said that in addition to the 800 federal troops, other reinforcements in support of local police were being directed to Rio amid fears that the crisis could spill over to neighboring states.