Striking police officers were embroiled in a mass-melee with hundreds of their own colleagues in riot gear who policed their protest, amid bizarre scenes in the Brazilian city of Sao Paulo.

The clashes between state police and plainclothes investigators last night came after the demonstrators tried to break through a barrier protecting the state government palace. Officers fired shots, tear gas and shock bombs, and the scuffles broke out.

Critics will highlight the incident as another example of the chaotic and dysfunctional nature of policing in Brazil. Last year, the UN pointed out that very low salaries - over which officers are currently striking - encourage widespread corruption, with many police units forming their own vigilante groups, death squads and militias.

It also sharply criticised Brazilian police for major human rights violations, pointing out that many of the 694 deaths caused by officers between January and June 2007 in Rio were likely to have been extra-judicial killings.

Officers are also known to engage in gunfire with Rio’s heavily armed drug gangs. Innocent civilians are often caught in the crossfire.

Sao Paulo's Albert Einstein Hospital, located a few blocks from the government palace where the latest clashes took place, said in a statement it treated 13 people who were injured. It did not say whether they were state police officers or investigators.

Brazilian states have two separate law enforcement groups - a uniformed police force in charge of maintaining public order, and the so-called civil police who are plainclothes investigators. In some cities there are also municipal guards in charge of protecting public property.

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