A demonstrator peeks out from his tent during a protest calling for the impeachment of Brazil's president Dilma Rousseff in Brasilia (AP)

The lower chamber of Brazil's Congress has begun debate on whether to impeach President Dilma Rousseff.

The atmosphere in the lower Chamber of Deputies was electric at the start of the session, with the crucial vote set for Sunday.

Those backing impeachment allege Ms Rousseff's administration violated fiscal rules. They say the government used sleight of hand accounting in a bid to shore up public support.

However, many of those pushing for impeachment face accusations of corruption themselves.

Ms Rousseff's defenders insist she did nothing illegal, and say similar accounting techniques were used by previous presidents.

If 342 of the lower house's 513 legislators vote in favour of impeachment, the process moves to the Senate.

Miguel Reale Junior, author of the impeachment petition, said Ms Rousseff's manoeuvring directly led to the ills plaguing the country, such as high inflation and periodic devaluations of the Brazilian real against the US dollar.

"Are you going to tell me that isn't a crime?" Mr Junior told the legislators, adding that the impeachment push was not "a coup", as government supporters contend.

Solicitor general Jose Eduardo Cardozo argued that deputies should only consider the accusations against Ms Rousseff.

He warned that impeachment would constitute an act of "violence without precedent" against democracy and the Brazilian people.

Flanked by people holding signs showing the constitution being ripped apart, Mr Cardozo insisted the impeachment process was an act of personal vengeance against Ms Rousseff by house speaker Eduardo Cunha.

He said Mr Cunha was striking out at Ms Rousseff for refusing to help him avoid an ethics probe into allegations he received millions in bribes from the sprawling corruption scheme in the Petrobras oil company.

"Violence has been committed against the democratic state," Mr Cardozo shouted, gesticulating wildly.

The drama comes as Brazil faces problems on many fronts: the economy is expected to contract nearly 4% this year, the Zika virus, which causes birth defects, has become a health crisis in poor, north-eastern states, and the country is less than four months away from hosting the Olympic Games.

The extraordinary session began hours after the Supreme Court denied a government motion to annul the impeachment proceedings. Mr Cardozo had argued that the process had been "contaminated" because legislators were considering things that went outside the accusations, such as the country's worst recession in decades and the Petrobras scandal.

Both government and opposition forces say they have enough votes to win on Sunday, but daily counts by Brazilian media suggest the opposition is much closer to victory.