Drug traffickers are behind the wave of violence that has affected Sao Paulo, Brazil's largest city, over the past few weeks, Sao Paulo state Gov. Geraldo Alckmin said.

"They are attacking the police to divert attention and relieve the pressure we are putting on drug trafficking, but we are not going to retreat one millimeter," Alckmin, a member of the opposition Brazilian Social Democratic Party, or PSDB, said.

Sao Paulo police killed eight suspected criminals between Friday night and Saturday morning, officials said.

The killings were the result of "resistance followed by death," the Sao Paulo Public Safety Secretariat said, adding that an internal investigation of the shootings was being conducted.

An investigation was opened following the killings of eight other people in Osasco, a city in the Sao Paulo metropolitan area, by gunmen wielding large-caliber weapons.

The killings occurred as fans of the Palmeiras soccer team were celebrating the club's Copa de Brasil win.

Investigators suspect that the killings may have been committed by security forces members seeking to avenge the murders of seven police officers in recent days, as well as four attacks on police stations and the torching of 13 buses. EFE