Police in Brazil are preparing for protests in the capital, Brasilia, ahead of a congressional committee vote later on Monday.
The 65-member committee will vote on whether to recommend the impeachment of President Dilma Rousseff over allegations she manipulated government accounts to hide a growing deficit.
Ms Rousseff denies the allegations.
A two-metre-high (6.5ft) metal barricade is being built to keep anti- and pro-government protesters apart.
The 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.
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But analysts say it will serve as an indicator of how key members of the lower house feel about impeaching the president.
Brazilian media are reporting that more than 100 of the 513-member lower house are still undecided.
513 members of the lower house of Congress
342 votes needed for her suspension
172 votes needed to block her impeachment
180 days she would be suspended for while the Senate debates her impeachment
If Ms Rousseff can get 172 members to vote against her impeachment, the proceedings will be shelved.
If, however, 342 vote in favour of the impeachment, Ms Rousseff will be suspended for 180 days while the matter goes to the upper house, the Senate.
There, the final decision on Ms Rousseff’s impeachment will be taken.
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.
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.
Ms Rousseff’s nomination of Lula triggered clashes between pro- and anti-government supporters.
Brazilian security forces say they are determined to keep the two sides apart this time.
They have instructed anti-government protesters to keep to the south side of the barrier and supporters of the government to stay to the north side.