Brazil is awaiting the outcome of a congressional committee vote – a key step in the process to impeach President Dilma Rousseff.
The 65-member committee will decide whether to recommend impeachment over allegations she manipulated government accounts to hide a growing deficit.
Police in Brazil are preparing for protests in the capital, Brasilia.
A two-metre-high (6.5ft) metal barricade is being built to keep anti- and pro-government protesters apart.
The committee began its deliberations at about 11:00 local time (14:00 GMT), O Globo reported.
Attorney General Jose Eduardo Cordozo, speaking for the president at a bad-tempered meeting, condemned the “flawed” process.
“It is absurd to dismiss a president who has not committed crimes, nor stolen a penny. And such a process without crime or fraud, would be a coup,” he said.
But the committee’s head, Jovair Arantes, who last week said there were grounds for the impeachment process to continue, on Monday defended that decision, saying it had been backed by lawyers, economists and the media.
The committee’s vote is largely symbolic, as no matter what the outcome of it is, the full lower house of Congress will vote on the impeachment later this week or at the beginning of next.
But analysts say it will serve as an indicator of how key members of the lower house feel about impeaching the president, and media suggest the committee’s vote could be close.
Brazilian media are reporting that more than 100 members of the 513-strong lower house are still undecided.
If Ms Rousseff, who strongly denies the allegations against her, can get 172 members to oppose the impeachment, the proceedings will be shelved.
If, however, 342 vote in favour of the impeachment, the matter will go to the Senate.
The Senate will then have to decide whether to proceed with the impeachment. A simple majority would suffice for that.
If that happens, Ms Rousseff will be suspended for 180 days while the impeachment trial continues in the Senate.
For her to be removed from office permanently, two-thirds of the Senate will have to vote in favour.
The security forces said they were expecting mass protests to coincide with the vote in the lower house with smaller demonstrations in the days preceding it.
Prisoners helped the security forces build an 80m-long metal barricade on the esplanade in front of the congressional building to keep supporters and opponents of the government apart.
Protesters have also been told to refrain from bringing inflatable dolls to the demonstrations and from wearing masks or otherwise obscuring their faces.
Dolls of former President Lula wearing prison clothing are a common sight at anti-government protests.
Brazilians are divided into those who support the government and who say the impeachment process is a coup d’etat against Ms Rousseff and those who allege that she and her predecessor in office, Lula, are corrupt.
The latest Datafolha opinion poll suggests 61% favour impeachment, down from 68% in March.
Lula is under investigation for alleged money laundering, which he denies.
Brazil’s Supreme Court is expected to rule this week on whether Lula can take up the post of chief of staff to Ms Rousseff.
Lula was sworn in to the post last month but was suspended almost immediately by a judge because of the allegations against him.