Brazil's lower house of Congress has opened a three-day debate whether to impeach President Dilma Rousseff on charges of breaking budget laws, ahead of the body’s impeachment vote that will take place on Sunday.
The government lost its last possibility to escape the vote when its request to halt it was rejected by the Supreme Court.
Rousseff is accused of manipulating budget accounts in 2014 to secure her re-election.
She has strongly rejected the accusation and planned to appeal to Brazilians in a televised speech on Friday night. But the increasingly isolated leader cancelled the broadcast after an opposition party sought a court injunction to block it, arguing that she was unfairly using resources of the Brazilian state to defend herself.
Police stepped up security in the Brazilian capital where a half-mile-long (1 km) metal fence has been erected on the grass esplanade opposite Congress to avoid clashes between rival demonstrators expected to turn out by the tens of thousands over the weekend.
In Rio de Janeiro, police said they plan to form a cordon on the Copacabana beachfront avenue to separate the pro-impeachment crowd from Rousseff supporters.
"I am very worried that there will be violence, depending on the result of the vote and the number of people who gather in Brasilia," said Congressman Rogerio Rosso, who chaired the lower house committee that backed Rousseff's impeachment.
The country's top network TV Globo plans to broadcast Sunday's critical roll-call vote from beginning to end, starting at 2 pm (1700 GMT), which analysts said will add pressure on lawmakers to vote for impeachment.
Polls show that roughly two-thirds of Brazilians support Rouseff's impeachment.
As opposition congressmen called for Rousseff's ouster, Attorney General José Eduardo Cardozo addressed Congress in her 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:0 "There won't be a coup."
While the budget violations alleged against Rousseff are serious, she has not been directly implicated in the kickback scandal engulfing state-run oil company Petrobras, though her opponents say that bribe money was used to fund her election campaigns.
Nineteen of the 25 parties with seats in the lower house now back impeachment, the Brasilia-based consultancy Arko Advice said on Friday. They will deliver at least 350 votes and maybe 370, exceeding the two-thirds majority in the 513-seat house needed to send impeachment to the Senate, it 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 disobeying budget laws.
If the Senate approved a trial, in a vote that would take place on May 11, Rousseff would automatically be suspended and replaced by Vice President Michel Temer.
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. Lower house speaker Eduardo Cunha who is the next in line to assume the presidency after Temer is also among those under watch.