Unions and backers of President Dilma Rousseff marched Friday in several cities in Brazil, mostly to show support for state-run oil company Petrobras as it is engulfed by a corruption scandal, but also to bolster the leader.

Most of the peaceful demonstrations drew less than 1,000 people by midafternoon, according to police and local news media, though authorities estimated several thousand gathered in Sao Paulo's central area by early evening.

The demonstrations came ahead of planned anti-Rousseff protests planned for Sunday, which have garnered much interest on social media.

Petrobras is at the center of what federal prosecutors call the biggest corruption scheme yet uncovered in Brazil. Investigators say the nation's biggest construction and engineering firms paid at least $800 million in bribes and other funds to Petrobras executives and politicians in exchange for inflated contracts with the oil firm. Some of the money was allegedly funneled into the campaign coffers of the Workers' Party and its allies.

The scandal has sent approval ratings for Rousseff's government to an all-time low of 23 percent in recent polling. There have also been calls from some corners to wrest control of Petrobras from the government's hands and further privatize the company, which now is state-controlled but has private investors and shareholders.

"We're all here to defend Petrobras from those who are using the corruption scandal to demoralize and privatize it. This is something we do not accept and will fight to prevent it from happening," said Natael Mendes, a metalworker and union member who gathered with others on a main Sao Paulo avenue. "The scandal must be investigated and those guilty must be punished. But it cannot be used to justify the destruction of Petrobras."

Another marcher in Sao Paulo, Lais Mendonza, a seamstress, said that she was at the demonstration to back Rousseff for what her Workers' Party had done to help poor Brazilians since it took power in 2003.

"She and the PT are being attacked from all sides by the wealthy who cannot accept what they have done to help the poor," Mendonza said.

___

Associated Press writer Brad Brooks in Rio de Janeiro contributed to this report.