Some areas were rocked by intense gun battles as drug gangs and local militias fought for territory.

Armed gangs are increasingly moving into the security vacuum, charging people to protect them from drug traffickers, correspondents say.

More than 50 people have been killed in Rio so far this month, a new independent monitoring group says.

The latest violence was concentrated mainly in the Barbante and Vila Juaniza favelas, or slums, where drug gangs were trying to re-take control from local militias, police said.

Local businesses and people have increasingly been turning to the armed militias for protection, who in turn impose their own form of order and extortion, correspondents say.

A police spokesman said they would investigate the alleged involvement of security officials in the militias.

Body count

Rights group Amnesty International has said the rise of the militias represents a failure to guarantee public safety.

A newly formed watchdog called Rio Body Count, which says it was inspired by the Iraq Body Count group, is trying to keeping track of reported violent deaths and injures.

On its website, the group reports that more than 50 people have died in the metropolitan area of Rio de Janeiro since 1 February.

The site's aim is to provoke discussion and put pressure on the authorities to act, the group says.

One of Rio Body Count's founders, Andre Dahmer, told the BBC News website via e-mail that their hope was to see a lasting peace and social justice in Rio, not a peace imposed by force of arms.

The group rejects accusations that its site portrays a negative image of Rio, stressing that their role is to collect information.

Last month, the governor of Rio de Janeiro state requested the deployment of federal security forces to help tackle rising violence.

Rio is gearing up for its world-famous carnival in a couple of weeks.