Paramedics told media the man fired a revolver and a .38-caliber pistol inside the cathedral at 1:00 pm (1500 GMT) as mass was going on, also wounding several people before killing himself.

The motive of the shooting and the identity of the gunman were not immediately known.

"An individual entered a church and opened fire on several people," a police spokesperson said.

"Currently the information we have is six deaths and three wounded," the spokesperson said, declining for the moment to say whether the gunman was included in the toll.

However television images showed the lifeless body of the gunman, wearing a blue t-shirt, lying inside the cathedral, holding a discharged pistol in his right hand, and a revolver tucked into his jeans.

Paramedics were also seen treating several people outside the cathedral and there were multiple police cars in the street.

"Everybody ran out. You can imagine that I'm shocked," one woman who was attending the mass told Globo TV.

The archdiocese of Sao Paulo said on its Facebook page that the killings provoked "deep pain" and added that the cathedral would be closed for as long as required for the police investigation.

A police officer told Globo TV that most of those in the cathedral at the time were elderly.

"He opened fire on innocent people. It's a big tragedy," the officer said.

Brazil is one of the most violent countries in the world. Last year it registered a record 63,880 murders. Its homicide rate of 30,8 per 100,000 inhabitants is three times higher than the level the UN considers to meet the definition of "endemic violence."

Seventy percent of murders in Brazil are committed with firearm, according to a monitoring group, Forum for Public Security.

The country's incoming president, Jair Bolsonaro, who takes office on January 1, easily won November elections on a platform that included promises to ruthlessly crack down on crime and ease gun laws to allow "good" people to defend themselves.