Brazil's lower house of Congress is voting on whether to impeach President Dilma Rousseff over charges of manipulating government accounts for political gains.
The "yes" camp was leading by a wide margin after more than half of the legislators had cast their ballots on Sunday evening. To succeed and be sent to the Senate for approval, the motion needs a two-thirds majority - or 342 votes.
Thousands of pro and anti-impeachment protesters gathered in the capital Brasilia and other cities as the vote got under way.
The 513 legislators voted one by one, with all MPs given time to speak before casting their votes.
Al Jazeera's Lucia Newman, reporting from Brasilia, said protesters from rival sides were watching the voting process outside Congress.
"We are seeing very pessimistic faces among the pro-government protesters who are watching the vote on large television screens," she said. "Pro-impeachment protesters are cheering every 10 seconds or so when the 'yes' vote is cast. A dire moment certainly for Brazil's embattled president."
As Brazilians hold their breath awaiting a crucial Congress vote that will have profound consequences for the future of President Dilma Rousseff and their country, a nagging question remains.
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Is the embattled president facing possible impeachment for allegedly tampering with public accounts to hide a huge budget deficit ahead of her narrow re-election in 2014?
Or, is she paying the price for presiding over the biggest economic crisis in Brazil since the Great Depression, coupled with the worst corruption scandal in the country's history?
The rival protesters were separated by a metal barrier and security forces searched them before letting them in either of the camps.
In Rio de Janeiro, which is scrambling to organise the Olympics in August, the two sets of protesters demonstrated at separate time slots on Copacabana beach.
"There won't be a coup, there'll be a fight," one woman dressed in the red of Rousseff's Workers' Party shouted.
READ MORE: Dilma Rousseff caught in media firestorm
Rousseff, 68, is accused of illegal accounting manoeuvres to mask government shortfalls during her 2014 re-election.
Many Brazilians also hold her responsible for tanking the economy and a corruption scandal centred on state oil company Petrobras - a record that has left her government with 10 percent approval ratings.
Rousseff accused her vice president, Michel Temer, and the house speaker of "treachery" and coup-plotting.
She also pledged to "fight until the last minute ... to foil this coup attempt".
If the vote passes, the Senate will vote, probably in May, whether to open a trial.
In case of a yes vote, which experts consider likely, then Rousseff would step down for 180 days.
During this period she would be replaced by Temer. If the Senate then ended the trial with a two-thirds majority in favour of ejecting her, Temer would stay on until elections in 2018.
|Dilma Rousseff, 68, is accused of illegal accounting manoeuvres to mask government shortfalls during her 2014 re-election [Eraldo Peres/AP]
Source: Al Jazeera and agencies