RIO DE JANEIRO - Brazilians cast their votes on Sunday in the first round of their country’s most polarised election in decades, with leftist Luiz Inacio Lula da Silva expected to beat right-wing incumbent Jair Bolsonaro.

Most opinion polls have shown Lula with a solid lead for months, but Mr Bolsonaro has signalled he may refuse to accept defeat, stoking fears of institutional crisis or post-election violence.

A message projected on Rio de Janeiro’s Christ the Redeemer statue ahead of the vote read: “Peace in the Elections.”

Most surveys favour Lula, who was president from 2003 to 2010, by 10-15 percentage points. If he wins more than 50 per cent of valid votes, which several pollsters show within reach, that would clinch an outright victory, foregoing a second-round vote.

Decked out in Lula stickers, Adriana Schneider was voting at a primary school in Rio de Janeiro. The university professor, 48, said Bolsonaro’s administration had been “catastrophic” for investment in culture, arts, science and education.

“We’re living under a barbaric government,” she said.

The figures put Mr Lula on the cusp of the score needed to win outright and avoid a runoff on Oct 30: Half the valid votes, plus one.

But Mr Bolsonaro, known for his combative style, has repeatedly said "only God" can remove him from office, attacked supposed fraud in Brazil's electronic voting system, and vowed his re-election bid can have just three outcomes: "Prison, death or victory."

Mr Lula, the charismatic but tarnished former president seeking to stage a comeback at 76, says he fears the incumbent will create "turmoil" if he loses - a concern heard often in Brazil heading into election day.

Mr Bolsonaro's attacks on the voting system have raised fears of a Brazilian version of the riots that erupted at the United States Capitol last year after his political role model, former president Donald Trump, refused to accept his election loss.

Voting in Sao Bernardo do Campo, Lula acknowledged the dramatic turnaround in his fortunes after a conviction that he says was politically motivated.

“It’s an important day for me,” he said. “Four years ago I couldn’t vote because I was the victim of a lie... I want to try to help my country to return to normal.”

Bolsonaro voted in Rio, and said he expected to win the election in Sunday’s first round, despite his poor showing in surveys. The former army captain does not trust the pollsters, saying their results do not correspond with the support he sees at his campaign events.

“If we have clean elections, we will win today with at least 60 per cent of the votes,” Bolsonaro said in a video posted on his social media before voting. “All the evidence we have is favourable to us. The other side has not been able to take to the streets, has not campaigned, has no acceptance, no credibility.”

A winner could be announced within hours after polling stations close at 5pm in Brasilia (2000 GMT). If no candidate wins more than half of the votes, excluding blank and spoiled ballots, the top two finishers go to an Oct 30 run-off, prolonging the tense campaign season.

"I do think (Bolsonaro) will contest the election result if he loses," said political analyst Adriano Laureno of consulting firm Prospectiva.

"But that doesn't mean he'll succeed. The international community will recognise the result quickly... There might be some kind of turmoil and uncertainty around the transition, but there's no risk of a democratic rupture."

Supreme Court Justice Alexandre de Moraes, who is currently head of the federal electoral court, said he expected a peaceful day with “tranquil” results.

Moraes – who has been one of Bolsonaro’s main foes, leading investigations into him and his allies – tweeted that the electoral court “continues to work so that we all have a peaceful and calm Sunday”.

Observers from the Organization of American States, the Carter Centre, the Inter-American Union of Electoral Organisations (UNIORE) and other international bodies will be monitoring the vote.

The White House meanwhile said the US would be watching the vote "closely."

More than 500,000 security force members will be deployed on election day. Public Security Minister Anderson Torres sought to downplay fears of unrest.

"We're having an election, not a war," he said.

Mr Lula, the former metalworker who rose from destitute poverty to become the most popular president in Brazilian history, is seeking to stage a remarkable return, four years after falling spectacularly from grace when he was jailed for 18 months on controversial corruption charges.

Accused in a massive graft scheme centred on state-run oil company Petrobras, Mr Lula regained the right to run for office last year when the Supreme Court annulled his convictions, ruling the lead judge in the case was biased.

In the meantime, Mr Bolsonaro, 67, who swept to office on a wave of anti-establishment outrage in 2018, has lost his outsider shine.

Vowing to defend "God, country and family", the President retains the die-hard backing of his "Bibles, bullets and beef" base - Evangelical Christians, security hardliners and the powerful agribusiness sector.

But he has lost moderate voters with his management of the weak economy, his vitriolic attacks on Congress, the courts and the press, a surge in destruction in the Amazon rainforest, and his failure to contain the devastation of Covid-19, which has claimed more than 685,000 lives in Brazil.

Many voters are deeply disillusioned with both contenders - and the lack of other options - in a race where none of the other nine candidates managed to break out of single digits in the polls.

"It's like we have a gun to our heads," 27-year-old Uber driver Matheus Fernandes told AFP in Mr Lula's home state of Pernambuco.

His plan: Cast a blank ballot.

The polls open at 8am and close at 5pm (7pm Sunday to 4am Monday in Singapore), with results expected some two hours later.

Brazil's 156 million voters will also be electing the Lower House of Congress, one-third of the Senate and governors and state legislators in all 27 states.

Lula and Bolsonaro have both promised more generous welfare spending next year, adding to pressure on the federal budget and leading both to look at alternatives to current spending rules.

The newly established autonomy of Brazil’s central bank and Lula’s choice of a centrist former rival as running mate have reassured some investors that he would not trigger a disruptive break in economic policy.

Lula has vowed to make a sharp departure from Bolsonaro’s environmental policies after deforestation in the Amazon rainforest hit a 15-year high. hE has pledged to combat logging, step up protection of the biome and local tribes, and make Brazil a protagonist in climate diplomacy. AFP, REUTERS