Brazilian lawmakers launched an impeachment debate Friday that could topple President Dilma Rousseff, in a political crisis threatening to destabilize the country months before it hosts the Olympics.

The sessions will lead to a vote on Sunday that could take the 68-year-old leftist leader closer to being driven from office as Latin America's biggest economy struggles through a recession.

Police tightly guarded the area as lawmakers arrived at Congress, which was surrounded with metal barriers in case of disturbances.

House Speaker Eduardo Cunha predicted an intense debate.

"It's calm but I don't think it will continue that way until Sunday," he told reporters.

"It will go on till Sunday night and probably beyond for people who signed up (to speak). This is a historic process, there's no doubt."

Cunha is one of the leaders of the push to remove Rousseff despite facing corruption charges himself.

The debating chamber was packed with reporters while deputies for and against Rousseff held up signs reading "Impeachment now" or "Impeachment without a crime is a coup."

Late Thursday, the Supreme Court rejected her last-minute bid to have the impeachment proceedings suspended.

If the lower house votes to push on with impeachment, as seems increasingly likely, Rousseff will be sent to trial in the Senate in a process expected to last months.

Rousseff is accused of illegal government accounting tricks. The accusations, along with Brazil's deep recession, have dragged down her popularity.

Latest counts of voting intentions in the lower house by Brazilian newspapers show the pro-impeachment camp at or near the two-thirds majority of votes needed to pass the impeachment motion up to the Senate.

Rousseff vowed on Thursday: "The government will fight until the last minute of the second half... to foil this coup attempt."

She appealed to the Supreme Court alleging that the case against her was a vendetta marred by irregularities.

But in an emergency late-night session, justices turned down the petition for an injunction.

Rousseff has accused her vice president, Michel Temer, and the house speaker of "treachery" and coup-plotting.

She has promised to compromise with opponents if she is allowed to stay in office.

But the collapse of her coalition has been relentless, starting with the defection of Temer's PMDB.

"We have come to a decisive phase in this process and the best thing for the country is to have a result on Sunday, whatever it might be," said Leonardo Piciani, PMDB congressional leader.

"We have to turn the page and not waste more time in political battles and disputes," he told AFP.

Temer will take over as interim president if the trial starts in the Senate. Rousseff would have to step down for six months during the proceedings.

If the Senate then voted by a two thirds majority to impeach her, she would be ousted and Temer would remain in the presidency.

Rousseff's supporters have vowed to demonstrate in Brasilia each day from Friday. Pro- and anti-Rousseff rallies are also planned in other cities on Sunday.

Lawmakers were facing fierce lobbying ahead of the vote, including from Rousseff's top ally and predecessor as president, Luiz Inacio Lula da Silva.

But he, too, faces pressure: the courts have suspended his appointment as Rousseff's chief of staff over a corruption case against him, linked to a graft scandal at state oil company Petrobras.

Lula rallied support for Rousseff in a video message on Friday.

"We are going to defeat impeachment and end this crisis once and for all," he said.

Solicitor general Jose Eduardo Cardozo said the government was considering further judicial appeals.

"You cannot remove a president of the republic based on some pretext," he told a news conference.

"That creates instability for the future which the world is watching."