As revellers dress up in glittery costumes for a week of singing, drinking and dancing in Rio de Janeiro, many people will be celebrating Carnival under the barrel of a gun.

Rio's Carnival, famed the world over for lascivious behaviour and virtually naked beauties, takes place this year amid an explosion of criminal violence that has exposed the dark side of the "Marvelous City."

The latest surge began in December, when gangs torched buses and attacked police posts. A national security force has been sent to the city and in recent days police and drug traffickers have battled in the teeming slums, or favelas. At least 15 people have been killed in the past week in clashes.

The crackdown has intensified in the days leading up to Carnival, which started yesterday, with police occupying some of the favelas that are in the grip of drug gangs and vigilante groups.

The well-known leader of the Salgueiro samba school and his wife were slain execution-style after leaving a Carnival rehearsal on Tuesday. Public outrage came to a head this week after a six-year-old boy was killed, dragged through the city under a speeding vehicle by carjackers.

"For the past three years, I left the city with my family during Carnival," said Sara Castro, who designs displays for store windows. "In the past, I paraded for the Mocidade samba school but the city is too violent and we've even considered moving out once and for all."