RIO DE JANEIRO — The police will invade 40 of the most violent slums in this city before the 2014 soccer World Cup being held in Brazil, with the goal of establishing a permanent policing presence in communities now controlled by well-armed drug gangs, Rio State officials say.

The plans include occupying Rocinha, one of the city’s largest and most fortified slums, in what crime experts here say could be a huge and bloody battle that could define efforts to squeeze out gangs that have plagued the city for three decades.

The campaign is an expansion of a police “pacification program’’ that began in late 2008. It comes as Brazilian officials are feeling the weight of international scrutiny after being chosen to host both the World Cup and the 2016 Olympic Games. The police forces, in particular, have been reeling from some embarrassing setbacks.

In October, just two weeks after Rio was awarded the Olympic Games, a weekend of bloody confrontations between rival drug factions left 12 dead, including two police officers killed when gang members with heavy-caliber weapons shot down their helicopter. The body of one young man was found crumpled in a grocery store shopping cart left on a busy street.

Then in December, Human Rights Watch released an extensive report detailing Rio’s history of extrajudicial killings by the police. The report said that a substantial portion of the 2,467 suspected “resistance” killings in Rio State in 2007 and 2008 were unlawful, and that police officers were rarely brought to justice in the slayings.

Human rights experts said they were concerned that the planned police invasions would result in many more summary killings by the police.

“Rio needs to find a way to control not just its violent gangs, but also its police,” said Daniel Wilkinson, the deputy director of the Americas for Human Rights Watch. “If it tries to do one without the other, then this pacification program is almost certainly going to result in a bloodbath.’’

The police say the pacification program is meant to bring order to slums that overhang Rio’s wealthiest areas in the south and west of the city, where the bulk of the Olympic contests will be held. Heavily armed gangs control hundreds of neighborhoods in Rio and are largely responsible for the metropolitan region’s having one of the highest murder rates in the hemisphere, at nearly 35 for every 100,000 residents.

Rio State officials say they are focusing their efforts on the slums where gangs have the most dangerous guns, which allow them to terrorize residents and repel police incursions.

“These 40 slums that we have chosen are the arms, legs, trunk and brains of drug trafficking in Rio de Janeiro,’’ said Dirceu Silviana, spokesman for the Secretariat of Public Security, which oversees the police in Rio. “But the primary objective is not the drug trafficking, it’s to do away with weapons of war.’’

Rio officials say they will eventually expand the program to 100 slums, but would not give a timetable.

For the next four years, at least, the plan is to occupy an average of 10 communities a year.

Rio officials say that part of the campaign is trying to reduce police lethality and that officers who choose to work in the slums are being given special human rights training and bonuses that nearly double the entry level police salary of about $620 a month.

Since starting the program in November 2008, the Rio police have taken over nine communities totaling about 120,000 residents, officials said. That is a small portion of the 600 or so slums that have serious problems with drug trafficking and militias; the slums contain about one million people, officials said.

The program involves placing a large police contingent in a slum area on a permanent basis to interact with residents and keep traffickers from reinvading and acting as a “parallel power,” said Sérgio Cabral, the governor of Rio State. Once they have occupied a slum, the police will handle security full time.

So far the “pacification” police have been installed in small neighborhoods, like Dona Marta, which has about 6,000 residents. “The testimonials I have received from the people that have been freed from this parallel power are just incredible,’’ Mr. Cabral said. “ ‘We are now free from terrorism,’ they tell me. ‘Finally, governor, I can sleep at night.’ ’’

Mr. Silviana, the public security spokesman, said: “The timing and decisions about which locations we will go into will depend on our ability to train new police officers. Each occupation is a learning experience.’’ (NYT)