Striking police detectives in Sao Paulo have been involved in violent clashes with state troopers during a protest march over police pay.

Brazilian television news showed a melee as hundreds of state police in riot gear tackled plainclothes police investigators.

It came after the protesters tried to break through a barrier protecting the state government palace.

Officers fired shots, tear gas and shock bombs.

The Albert Einstein Hospital, located a few blocks from the government palace, said in a statement that it had treated 13 people who were injured in the clashes, none seriously.

It did not say whether those injured were state police officers or detectives.

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.

Detectives in Sao Paulo state have been striking for a month to demand better pay.