At least 10 people have been killed in a surge of violence that began with the murder of an off-duty policeman in the Brazilian city of Campinas. 

Reports put the death toll at 13 in violence overnight on Sunday, but a police spokesman said on Monday that only 10 deaths had been confirmed so far. 

"We have not established whether these deaths are related. All we know for now is that they occurred in the same area of Campinas," the spokesman told news agency AFP.

Some of the killings in Sao Paulo state's third largest city were perpetrated by criminals riding in vehicles, police said.

The news portal G1, quoting local police sources, said three of the victims were taken to hospital but died of their wounds, while the other 10 were shot dead.

The O Estado de Sao Paulo newspaper said the killings happened hours after an off-duty policeman was killed during a robbery attempt at a petrol station in the area.

No arrests have been reported. 

Last July, a survey on violence published by the Latin American Studies Centre said one million people were killed in Brazil between 1980 and 2011, making it the world's seventh most violent nation.

Source: AFP