Over 1,400 Brazilian marines and police officers entered a huge complex of slums in Rio de Janeiro on Sunday.
The armed operation is the latest security push by authorities in Brazil ahead of the World Cup this summer.
More than 130,000 people are believed to live in the Complexo do Mare, which consists of 15 slums and has been ruled by drug trafficking gangs for years.
Brazilian officials are keen to prevent violence in Rio’s favelas disrupting the World Cup, which starts on June 12.
At the weekend, armed officers and soldiers began patrolling the 5-square-kilometre complex.
The plan is to set up permanent posts in Mare as part of a “pacification” programme, which started in 2008 and officials hope will make Rio more secure before the World Cup this year and the Olympics in 2016.
Complexo da Mare is near Rio’s international airport, meaning visitors will pass near the slums on their way into the city centre.
The latest operation comes in what is reported to be a "critical" time for the security effort.
Gangs have attacked police outposts in other shantytowns in recent months.
The gang leaders are hoping to disrupt the police operations.
So far the pacification has seen some success, with the amount of violence in the slums said to be down.
However, there have been complaints of heavy-handed tactics by the authorities.