http://www.iepearl.comIn the most high-profile incident, the pilot of a military police helicopter was shot in the leg as he flew over the favela and the helicopter exploded in flames as it crash-landed on a nearby football pitch. Two of those on board were killed. It was the first time a police helicopter had been shot down in Rio.

Rio's mayor, Eduardo Paes, said it was "inadmissible that Rio be freshwater pearl confronted by delinquents in this way" and threw his weight behind police attempts to control the violence.

The head of the military police, Mario Sérgio Duarte, said the drug traffickers would "be the victims of their own choices". "We have lost two professionals who dedicated themselves to the defence of the population. But we will not be motivated by revenge," he added.

Oderlei Santos, spokesman for Rio's military police, said: "Our operations will only cease when these criminals are captured, arrested or are killed in combat."

Authorities cancelled all police leave and members of Rio's civil police gathered at the police HQ in central Rio this afternoon. They were expected to occupy a number of favelas around the city. Tonight, military police were seen entering at least one freshwater pearl jewelry slum controlled by the Red Command in Rio's southern beach district.

The latest round of violence underlines the challenges local authorities face as they attempt to improve security before the city hosts the 2014 World Cup and the 2016 Olympics. Rio's government has spent the past year expelling drug gangs and vigilantes from four slums and setting up "pacification" projects by which the slums are permanently occupied by police.

But the majority of the city's favelas are still controlled by members of three drug factions, which possess an increasingly sophisticated arsenal, including anti-aircraft guns and automatic rifles, often sourced from inventory intended for the Bolivian and Argentinian armies and smuggled into Rio.

Faced with an increasingly well-armed enemy, Rio's police are also investing heavily in military equipment. They now have a bulletproof helicopter, while local pearl jewelry wholesale journalists wear bulletproof vests when working in the slums. Each year, Rio's police kill around 1,000 people "resisting arrest". Nearly 90 officers have been killed this year.

Santos promised that things would improve before the Olympics. "We have a lot of time before the World Cup and the Olympics, and before then we will certainly arrest a lot of criminals," he said.