BRASÍLIA - Brazil's Senate on Wednesday launched a pivotal debate on the future of President Dilma Rousseff, who faces likely suspension and an impeachment trial that could end 13 years of leftist rule in Latin America's biggest country.

Even allies of Rousseff, 68, said she had no chance of surviving the vote, expected later in the day after a marathon session.

Brazil's Supreme Court on Wednesday rejected a petition from President Dilma Rousseff's government lawyers to halt impeachment proceedings against her, as the Senate debated suspending her from office and putting her on trial.

Justice Teori Zavascki "denied the petition to issue a stay suspending the process," a court spokesman told AFP, a decision that all but erases the leftist leader's hopes of avoiding a six-month suspension and trial in a Senate vote expected early Thursday. Rousseff is accused of illegal accounting maneuvers but says the charges are trumped up and amount to a coup by her center-right opponents.

A simple majority in the 81-member Senate would be enough to trigger Rousseff's six-month suspension pending judgment, in which a two-thirds majority would force her from office permanently.

Senate President Renan Calheiros, who was overseeing the proceedings, told reporters that impeachment would be "traumatic" for Brazil.

Numerous police were deployed near the building and streets were closed off, causing traffic jams.

Due to host the Olympic Games in less than three months, Brazil is struggling with the worst recession in decades and a corruption scandal that has ripped apart the political and business elite. "The process of impeachment... is long, traumatic and does not produce quick results," Calheiros warned.

He said he expected senators to vote on Wednesday night. Rousseff would be formally notified of her suspension on Thursday and he would discuss with her the details of how she would hand over power to her center-right Vice President Michel Temer.

Rousseff's government lawyer lodged a last-ditch appeal with the Supreme Court on Tuesday to block the vote, but the court had not even responded before senators sat down in their futuristic building in the capital Brasilia.

"There won't be any miracle. She'll be suspended for six months and then we'll open the debate on the merits" of the case, Paulo Paim, a senator from Rousseff's Workers' Party (PT), told reporters.

He said the impeachment drive was "a symbol of Brazilian politicians' incompetence, to accept a tainted process against a president they know is honest." But Magno Malta, a senator of the opposition PR party, said impeachment was needed to heal a sick country.

"As soon as we vote for impeachment, the dollar will fall (strengthening Brazil's currency), our stock market will rise and the patient will breathe again," said Malta.

"The doctor will say the patient is showing signs of life and is in intensive care," he added. "But it is a long, painful process which depends on all of us."

Rousseff is accused of breaking budgetary laws by taking loans to boost public spending and mask the sinking state of the economy during her 2014 re-election campaign.

She says the accounting maneuvers were standard practice in the past. She describes the impeachment as a coup mounted by Temer, who will take over if she is suspended. Rousseff vowed to resist.

"I am going to fight with all my strength, using all means available," she said Tuesday.

Rousseff called her opponents "people (who) can't win the presidency through a popular vote." She claimed they wanted to "dismantle" social gains made by millions of poor during 13 years of Workers' Party rule.

Rousseff's allies went to the top electoral court asking that Temer be barred from appointing his own ministers. But analysts say Rousseff's fightback probably comes too late and that she burned many of her political bridges before the crisis erupted with an awkward style and inability to negotiate.

The country's first female president has also become deeply unpopular with most Brazilians, who blame her for presiding over the recession and a massive corruption scandal centered on the state oil company Petrobras.

She took a morning walk on Wednesday outside the Planalto presidential palace, where she was expected to stay holed up all day.

Workers' Party faithful on Tuesday burned tires and blocked roads in Brasilia and in Sao Paulo in a potential taste of more street trouble to come.

Lawmaker Jose Guimaraes, a Rousseff ally, said that despite almost certain defeat in the initial Senate vote, the impeachment trial itself would be an all-out fight as his side fought to win over senators.

Police responded to heightened tension by building a huge metal barricade outside Congress in Brasilia to separate rival groups of protesters during the Senate vote.

A square where major government institutions are located will be declared a "national security zone" and made off-limits to the public, the Brasilia security authorities announced.

The Senate impeachment trial could last months, running through the Olympics, which open in Rio de Janeiro on August 5 - the first Games to be held in South America.