Brazilian police and soldiers late last year swept drug gangs out of some Rio slums in a high-profile and bloody operation, as the city prepares to host the 2014 World Cup and 2016 Olympics.

Preparations generally are proceeding on schedule ahead of the famous sporting events, as Brazil builds new stadiums and modernizes its infrastructure.

Going less well, however, are efforts to eradicate from Rio criminal gangs which have apparently decided that they will not be pushed aside without a fight.

Drug traffickers late Tuesday opened fire on soldiers in the Complexo do Alemao, a network of hillside favelas in northern Rio that was declared "pacified" after some 1,800 troops occupied the site in November 2010.

The shooting revived fears of gun battles that were common before the arrival of police and soldiers.

"It was terrifying, because we thought that nothing like this would happen again," tweeted Rene Silva Santos, 17, a neighborhood resident.

In response to the violence, authorities dispatched armored vehicles along with about 100 Marines. Some 200 more soldiers have been sent to the site, along with extra military police.

Military police also sent a 50-man force into the favelas on the Baiana and Adeus hills, to try gain control of the high ground in an area of the complex where no federal troops had been deployed.

Soldiers had already clashed with a group of Alemao residents on Sunday. Twelve people were wounded, including two soldiers, when the locals hurled rocks and bottles and troops responded by firing rubber bullets.

Endemic and chronic urban violence has long sullied the image of Rio, where a third of the population lives in 1,000 slums spread throughout the city.

Since 2008, authorities in Rio, which has one of the highest murder rates in the country, have been in a race to impose security in the city.

Last week, Rio authorities launched raids targeting so-called militia groups which see to establish their own security and protection operations in some communities.

At least 14 people were arrested from the squads, which are accused of murder, death threats, weapons violations and illegal fuel sales, among other charges, according to the Rio public security office.

Officials say the militias, often composed of current or former police officers, prison guards, firefighters and others, have taken over in some slums from drug gangs but use similar tactics including assassinations and collection of a "tax" for security.

The uptick of drug gang violence took officials by surprise.

"We knew that this could happen, but we nevertheless were taken by surprise," said Adriano Pereira Junior, commander of the military police, speaking about an assault by suspected drug traffickers.

The government's security chief, Jose Mariano Beltrame, declared during a press conference Thursday that 17 favelas with about 300,000 inhabitants have been "pacified."

But with respecting to keeping the notorious slums crime free, he said Brazil's law enforcement officials could not be expected to work "miracles."

"I've said repeatedly over the past 30 or 40 years -- there is no short term solution" to the problem of the drug traffickers in Brazil' favelas," Beltrame said.