Thousands protest against Brazilian President Dilma Roussef at National Congress in Brasilia, Brazil, on March 15, 2015. (Fernando Bizerra Jr/EPA)

Hundreds of thousands of people marched peacefully Sunday in more than 50 Brazilian cities to demand President Dilma Rousseff’s impeachment and to criticize government corruption.

The biggest protests were in Sao Paulo, an opposition stronghold where hundreds of thousands gathered on a main avenue, as well as in the capital, Brasilia. Dozens of cities had demonstrations involving a few thousand people each. According to the Web site of Globo TV, Brazil’s largest network, the total number in the streets across Brazil was more than 300,000 people, based on local police estimates.

Much protester ire was focused on a kickback scheme at the state-run oil company Petrobras. At least $800 million was paid in bribes and other arrangements by the nation’s biggest construction and engineering firms in exchange for inflated Petrobras contracts.

Top executives are in jail, and the attorney general is investigating dozens of top congressmen, along with current and former members of the executive branch, for alleged connections to the scheme, which apparently began in 1997. Rousseff’s party took power in 2003. Rousseff, a former chairwoman of Petrobras’s board, has not been implicated, though top officials from her administration are caught up in the inquiry.

The marches add pressure on Rousseff, whose poll ratings have never been lower and who is facing dual economic and political crises. But the protests are significantly different from the anti-government demonstrations of 2013. Those protests cut across political, social and economic lines, and were a widespread expression of frustration with poor public services, as well as a cry against government corruption.

Sunday’s protests, largely organized by ad-hoc right-leaning groups, appeared far more politically focused, and demonstrators came from social classes that voted against Rousseff in her narrow reelection victory in October.

Despite their partisan nature, the mass marches are another thorn in Rousseff’s side. They also add impetus to opposition efforts to thwart measures she backs in Congress, where even those in the ruling Workers’ Party coalition are creating hurdles as she tries to push through austerity measures.

Brazilian growth has been weak since Rousseff took office in 2011. The country entered a recession in 2014. Inflation is rising, and the local currency has plummeted against the dollar in recent weeks, making life more expensive.

Still, Brazil’s top opposition figures say impeachment is undesirable, because the president is not accused of any connection to the Petrobras scandal and because it could affect Brazil’s stability.