(Adds demonstrators trying to block Lula's motorcade)
By Marcela Ayres and Anthony Boadle
BRASILIA, April 16 (Reuters) - Brazilian President Dilma Rousseff held last-minute negotiations with wavering lawmakers on Saturday in an effort to secure crucial support the day before an impeachment vote that could lead to her removal from office.
The political crisis has divided the country and turned into a bitter clash between the leftist leader and her centrist Vice President Michel Temer, who would take over if she is unseated.
Rousseff canceled an appearance at an anti-impeachment rally of union and left-wing social activists led by former president Luiz Inacio Lula da Silva, her predecessor and founder of the ruling Workers' Party.
Instead she met with lawmakers behind closed doors in a bid to obtain their vote or abstention on Sunday when the lower house of Congress votes on whether she should be impeached for breaking the country's budget laws.
The talks indicated that Sunday's ballot may be tighter than expected as Rousseff seeks to swing the estimated two dozen additional votes she needs to prevent a two-thirds majority in favor of impeachment in the 513-seat lower chamber, which her opponents need to push ahead with this process.
A Temer aide said that Rousseff, with the help of Lula, who is still Brazil's most influential politician despite a graft probe, had managed to reverse a "handful" of votes but not alter the growing momentum for impeachment.
In a rowdy marathon session of speeches that went through the night, opposition congressmen shouted: "Out with the Workers' Party!" Rousseff's supporters, meanwhile, called for the ouster of House Speaker Eduardo Cunha, her political archenemy who has sped up the impeachment process.
Rousseff is fighting to survive a political firestorm fueled by Brazil's worst recession since the Great Depression of the 1930s and a spiraling corruption scandal centered on state oil company Petrobras that has reached her inner circle.
The president's opponents want her dismissed from office on charges that she manipulated budget accounts to boost public spending to boost her 2014 re-election. They also blame her for running Latin America's largest economy into the ground.
In a video and a newspaper column, Rousseff - Brazil's first female president - strongly denied she had committed an impeachable crime and called the bid to oust her "the biggest legal and political fraud" in the country's history.
"We are facing the threat of a coup d'état, a coup without guns that uses more destructive methods like fraud and lies to try to destroy a legitimately elected government," she wrote in the Folha de S.Paulo newspaper's Saturday edition.
Rousseff could become Brazil's first president to be impeached since 1992 when Fernando Collor de Mello resigned just before imminent impeachment for corruption. The current crisis is far deeper because he had few supporters while Rousseff has the backing of working class Brazilians who have seen their quality of live improve during 13 years of Workers' Party rule.
"If she loses and Temer takes over we will take to the streets, and we have the strength to stay there as long as necessary," said Marco Antonio Baratto, coordinator of the MST landless peasants movement, at a rally of several thousand Rousseff supporters camped out in the parking lot of Brasilia's football stadium.
A tired-looking Lula, the labor leader who led Brazil as president from 2003 to 2010, addressed the rally briefly with a hoarse voice and said he had to rush back to the talks with lawmakers.
Lula's bodyguards scuffled with impeachment supporters who tried to block his motorcade returning to his hotel.
If her impeachment is approved by the lower house, the Senate must then vote on whether to go ahead with putting Rousseff on trial for disobeying budget laws. If she loses that vote, which would likely take place on May 10 or 11, Rousseff would automatically be suspended and replaced by Temer.
Temer, who would serve out Rousseff's term until 2018 if she is found guilty by the Senate, has little popular support. He would face a daunting task restoring confidence in a country where dozens of political leaders, including his close associates, are under investigation for corruption.
Even if Rousseff wins Sunday's vote, her coalition has been left in tatters by the acrimonious impeachment process and analysts say her pledges to form a national unity government are unlikely to bear fruit in a bitterly divided country.
A case before the Supreme Electoral Court concerning charges of illegal funding of her 2014 campaign could eventually lead to both Rousseff and Temer being dismissed from office and new elections called.
In the video posted on social media on Friday, Rousseff warned that if she is toppled by the impeachment process, a Temer government would force Brazilians to make sacrifices as well as roll back social advances that have reduced poverty during 13 years of Workers' Party rule.
The vice president, who has been preparing to form a new government aimed at restoring confidence and economic growth, replied on Twitter that the accusation that he will cut social programs for the poor was an "abject lie."
Temer is considering the chairman of Goldman Sachs in Brazil, Paulo Leme, and the founder of asset manager Mauá Capital, Luiz Fernando Figueiredo, as candidates to join his economic team should he take over the presidency, two people familiar with the matter told Reuters on Friday.
Reporting by Marcela Ayres, Eduardo Simoes and Alonso Soto; Writing by Anthony Boadle; Editing by G Crosse and James Dalgleish