Right-wing candidate Senator Flavio Bolsonaro cast his vote early on Sunday morning, calling for 'real change' as Brazil heads to the polls.
The hotly-contested election will determine whether Brazil remains one of Latin America's few leftist nations or joins a regional swing to the right championed by US President Donald Trump.
'Brazil can't stand another four years like this, everyone's life has gotten worse,' Bolsonaro, who has vowed to crack down on crime and slash government spending, said outside a polling station in Rio de Janeiro.
The 45-year-old is up against veteran leftist President Luiz Inacio Lula da Silva, who has pitched himself as a bulwark against US interference and democratic backsliding with the promise to expand social protections.
The 80-year-old is hoping to pull off a dramatic turnaround after a shock second-place finish in the first round to the son of imprisoned former president Jair Bolsonaro.
Final polls showed the two candidates neck-and-neck.
It is the final electoral battle for Lula, who is seeking a fourth non-consecutive term. He told voters today that they were choosing between a candidate who defended the national flag or one who 'wants to govern with the American flag'.
Lula, revered for social welfare programs that lifted millions out of poverty, made a dramatic comeback to office in 2022 elections after a massive corruption scandal saw him jailed for over 18 months.
Right-wing candidate Senator Flavio Bolsonaro cast his vote early on Sunday morning, calling for 'real change' as Brazil heads to the polls
'Brazil can't stand another four years like this, everyone's life has gotten worse,' Bolsonaro said outside a polling station in Rio de Janeiro
President Luiz Inacio Lula da Silva has pitched himself as a bulwark against US interference and democratic backsliding with the promise to expand social protections
The charges, annulled on procedural grounds, have continued to haunt him.
In his third term he has overseen a drop in inflation and unemployment, and lifted millions out of hunger, but voters complain about the cost of living and a sense of insecurity.
Meanwhile Bolsonaro, the son of Trump's ally, has said he would hand Brazil to 'the Lord Jesus Christ,' with a message of change that won over Brazilians looking for something new.
'This is about renewal, enough of the same old thing,' 67-year-old retiree Eduardo Viana said, adding that Lula 'says he governs for the poor... just look, people can't even go to the supermarket.'
Bolsonaro presents himself as more moderate than his firebrand father, who is serving a 27-year sentence under house arrest for an attempted coup bid after his 2022 election loss.
He vows that, if elected, his first act will be to free Jair Bolsonaro, a deeply divisive figure in Brazil whose legacy loomed large over the campaign of his eldest son.
After its first-round defeat, the left swung into action with massive rallies declaring 'Never Again Bolsonaro,' where protesters recalled the former president's chaotic handling of the Covid pandemic, devastation of the Amazon rainforest and cuts to public programs.
Deforestation of the world's largest rainforest has hit its lowest level in four decades under Lula.
President Donald Trump with then-Brazilian President Jair Bolsonaro at a Mar-a-Lago resort in Palm Beach, Florida, 2020
People queue to vote at a polling station during the presidential runoff election, at Cidade de Deus favela in Rio de Janeiro, Brazil
A man wrapped in a Brazilian flag arrives to vote in the presidential runoff in Brasilia, Brazil
Yet, as the El Nino weather phenomenon fueled fires that choked Amazonian cities with smoke in recent weeks, climate change barely featured in the campaign.
Lula's famed social policies became a point of tension as he expanded welfare programs during the campaign. Critics want the government to rein in spending.
'Even if people call it handouts, for us, they were an enormous help. Because anyone who has ever gone hungry knows not to take these programs for granted,' said Walter dos Santos, 56, who lives in northeastern Brazil, a bastion of support for the president, who hails from the same region.
After his first-round blow, Lula revamped his social media strategy, turning to video selfies and even buying his first cellphone.
A fierce social media battle drew in Meta, parent company of WhatsApp, Facebook and Instagram, which Lula's campaign accused of favouring his opponent with 'algorithmic manipulation.'
Meta has denied this, but acknowledged that some of Lula's campaign ads had been mistakenly deactivated due to technical errors.
Whoever wins, Brazil is facing a bruising election aftermath.
'If the left wins, it faces four years with a hostile Congress and threats of impeachment,' said political analyst Thomas Traumann, referring to sweeping legislative gains by the right.
'If the far right wins, it will be a government of retribution, directed at the Supreme Court,' hated by the right after its conviction of Jair Bolsonaro over the attempted coup.
Flavio Bolsonaro has said he would seek the impeachment of Supreme Court Judge Alexandre de Moraes, who led the case against his father.
Polling stations will close at 5:00 pm local time (2000 GMT), and results are expected within hours.