SAO PAULO (AFP) - Ten public buses were destroyed on Monday after 12 people were shot and killed overnight in the Brazilian city of Campinas, police said.

The surge of violence began with the murder of an off-duty policeman, according to police and press reports.

A spokesman said police were investigating various possible motives for the shootings, "including revenge, conflicts between rival gangs or even police executions".

A group of about 20 assailants, most hem hooded and armed with sticks and stones, ransacked a Campinas bus terminal where five of the overnight deaths occurred. They set fire to three buses and ransacked seven others, according to police.

The local association of urban transport companies said the terminal had to shut down and traffic on seven bus lines was suspended for several hours as police intervened to restore order.

Some of the overnight killings were perpetrated by armed criminals riding in vehicles, police said.

"We have not established whether these deaths are related. All we know for now is that they occurred in the same area of Campinas," Sao Paulo state's third largest city, the spokesman noted.

The news portal G1, quoting local police sources, said three of the victims were rushed to hospital but died of their wounds while the other died on the scene.

The daily O Estado de Sao Paulo, meanwhile, said the killings occurred hours after an off-duty policeman was killed during a robbery attempt at a gas station in the area.

No arrests have been reported so far.

Campinas is located 90km from Sao Paulo, the state capital which will host the opening game of the World Cup on June 12.

In late 2012, the state of Sao Paulo was rocked by a wave of violence that left scores of people dead, including several police officers.

Experts attributed it to an undeclared war between a prison-based criminal gang known as the First Command of the Capital (PCC) and police.

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