Pro-impeachment lawmakers have chanted "Dilma Out" in the lower house of Brazil Congress as it opened a raucous three-day debate on whether to impeach President Dilma Rousseff on charges of manipulating budget accounts.
Pro-government demonstrators took to the streets in several states as the debate began on Friday (local time), according to local media.
Major trade unions and landless peasant movements planned bigger, nationwide protests on Sunday, when the debate is set to culminate with a vote that Rousseff is widely expected to lose.
The government lost a last-ditch appeal on Thursday before the Supreme Court to halt the impeachment process, which could bring further instability or even chaos to Latin America's largest economy.
Rousseff is accused of manipulating budget accounts in 2014 to secure her re-election. She has strongly rejected the accusation, branding the impeachment process "a coup."
The president, a former leftist guerrilla, was to address the nation in a televised speech later on Friday, a presidential source told Reuters.
As lawmakers called for her ouster, Attorney General Jos Eduardo Cardozo addressed the house in Rousseff's defence, calling the impeachment process a "violent act with no parallel against democracy."
"History will never forgive those who broke with democracy," Cardozo said, as ruling lawmakers shouted: "There won't be a coup."
While the budget violations alleged against Rousseff are serious, she has not been accused of committing any crime or participating in the massive corruption scandal engulfing state-run oil company Petrobras.
But she is highly unpopular and more than a million protesters called for her impeachment in protests last month. The move is widely seen as a vote of no-confidence in a leader who has presided over the country's worst recession in decades.
After several allied parties abandoned Rousseff's coalition in recent weeks, the opposition is expected to gain the two-thirds of votes in the 513-seat lower house to send the process to the Senate.
If Rousseff is ousted, it would end the 13-year rule of her leftist Workers' Party, which has lifted millions of Brazilians out of poverty and is overwhelmingly supported by the poor.
One of the authors of the impeachment filing, former justice minister Miguel Reale Jr, opened the debate by saying the process to oust Rousseff reflected the will of the people.
Polls show that roughly two-thirds of Brazilians support impeachment.
"She was extremely irresponsible and knocked out the country," Reale Jr said.
If her impeachment is approved by the lower house, the Senate must then vote on whether to go ahead with putting Rousseff on trial for breaking budget laws.
If it agrees to do so, in a vote expected on May 11, Rousseff would automatically be suspended and replaced by Vice President Michel Temer, pending a trial that could last six months.
Temer, who would serve out Rousseff's term until 2018 if she is ousted by the Senate, has little popular support. He would face a daunting task restoring confidence in a country where dozens of political leaders, including his close associates, are under investigation for corruption.