Brazil's lower house of Congress is due to hold a crucial vote on whether to impeach President Dilma Rousseff over charges of manipulating government accounts for political gains.
Security has been stepped up outside the building in Brasilia to prevent clashes between rival protesters.
Ms Rousseff has accused her political opponents are mounting a "coup".
She has been lobbying MPs amid indications there may be enough votes for the impeachment motion to carry.
A two-thirds majority — 342 out of 513 votes — are needed to send the motion to the upper house, the Senate, where a simple majority would be enough to suspend Ms Rousseff from office for up to 180 days during an impeachment trial.
The debate on impeachment began on Friday, and the voting is expected from 17:00GMT (1800BST). Deputies will vote one by one in a televised session which is expected to last several hours.
On Saturday, the president wrote in the Folha de Sao Paulo, (in Portuguese): "They want to convict an innocent woman and save the corrupt," a reference to charges several politicians face.
She accused her opponents of "fraud and lies in an attempt to dismiss a legitimately elected government and replace it with a government without legitimacy".
Denying she had committed any crime, she said: "This is a coup against the republic, against democracy and above all against the votes of all Brazilians who participated in the electoral process."
She later cancelled a planned appearance at a rally to contact MPs and political leaders.
A number of coalition parties, including the biggest, the PMDB, have already abandoned her to support the impeachment.
Ms Rousseff, 68, has accused her Vice-President, Michel Temer, of being one of the ringleaders of the "coup" attempt against her.
She said a widely distributed audio message of Mr Temer appearing to accept replacing her as president was evidence of the conspiracy. However, she did not identify him by name.
Ms Rousseff has also indicated lower house speaker Eduardo Cunha is among those trying to oust her. He is himself facing money-laundering and other charges.
What happens next?
Lower house vote: An impeachment vote is due in the lower house on Sunday. A two-thirds majority is required for it to go forward to the Senate.
Senate vote on trial: If Ms Rousseff case is sent to the Senate, a simple majority is enough to suspend her for up to 180 days while she is put on trial. Vice-President Michel Temer would step in during this period.
Impeachment vote: For Ms Rousseff to be removed from office permanently, two-thirds of the Senate would have to vote in favour. Mr Temer would remain president for an interim period should this happen.