At least 21 people have been killed and 153 arrested in four days of clashes between drug gangs and Brazilian security forces in Rio de Janeiro, the police said.
On Wednesday alone, 13 presumed drug traffickers were killed and two police officers wounded in police operations in 27 slums, known as favelas, police Colonel Lima Castro told reporters.
In the unrest 21 vehicles were set ablaze, he said.
The police in Rio used armored tanks backed by helicopters to launch the massive crackdown on gangs and drug trafficking in the city's favelas.
Castro said that some 17,500 officers had been mobilized for the operation, scheduled to continue on Thrusday.
Authorities say that two factions of drug dealers have joined forces seeking to disrupt a two-year-old favela pacification programme aimed at wresting the densely populated areas from the gangs' control.
The escalating violence raises questions on the ability of Rio authorities to get a grip on security ahead of its hosting of two of the planet's largest sporting events, the world cup in 2014 and the Olympics two years later.