Brazilian President Luiz Inácio Lula da Silva, nearly 81, has clinched a fourth term in a tight runoff against Flávio Bolsonaro.
The leftist leader, known monomynously as Lula, won 51.7 per cent of the vote, compared to 48.2 per cent for the son of former president Jair Bolsonaro, official results showed.
Brazil's election stood at a virtual tie halfway through counting on Monday, with Lula holding a razor-thin lead over the right-wing Bolsonaro.
It was the final electoral battle for Lula, who turns 81 on Tuesday, pulling off a dramatic turnaround after coming in second to Bolsonaro in the first round of voting.
After a campaign marred by scandal and mudslinging, Monday's vote plunged into chaos when a last-minute Supreme Court ruling extended voting by two hours at the request of Lula, who said buses had been pulled from circulation in various cities, barring citizens from voting.
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"I've never complained about anything in any election before. Whether I won or lost, I'd just accept it and get ready for the next one. But this time, something abominable is happening," said Lula.
Bolsonaro also requested an extension in some parts of the country due to heavy rains, but his camp lashed out at the country-wide extension.
"It's a disgrace. We've never had anything like this in Brazil, and now they've come up with the idea of extending the election period by two hours. I think it's an attempt to even things out, but Flávio is going to win," Aurelio Junior, a 45-year-old dentist in Brasilia, said.
Jacqueline Maria Lima de Franca, 34, a business owner in Rio de Janeiro backing Lula, said it was "more than fair ... everyone should have the right to vote, and that was being neglected".
Lula's win cements Brazil as one of Latin America's few leftist nations, refusing to join a regional swing to the right championed by United States President Donald Trump.
Lula, who will now serve a fourth non-consecutive term, pitched himself as a bulwark against US interference and democratic backsliding.
Trump, an ally of Flávio Bolsonaro's father, simply said "we are watching, yeah", when asked about Brazil's election on Sunday.
The 45-year-old Bolsonaro had vowed to crack down on crime and slash government spending, while handing Brazil to "the Lord Jesus Christ".
"The time has come for real change," he declared after voting in Rio de Janeiro.
"This is about renewal, enough of the same old thing," 67-year-old retiree Eduardo Viana told a journalist from the Agence France-Presse (AFP) news agency, adding that Lula "says he governs for the poor ... just look, people can't even go to the supermarket".
Bolsonaro presented himself as more moderate than his firebrand father, who is serving a 27-year sentence for plotting a coup after losing the 2022 elections.
After its first-round defeat, the left swung into action with massive rallies highlighting Jair Bolsonaro's chaotic handling of the COVID-19 pandemic, devastation of the Amazon rainforest and cuts to public programs.
Lula, revered for social welfare programs that lifted millions out of poverty, made a dramatic comeback to office in the 2022 elections after a corruption scandal saw him jailed for over 18 months.
The charges, annulled on procedural grounds, have continued to haunt him.
In his third term, he oversaw 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.
After his first-round blow, Lula revamped his social media strategy and even bought 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 denied this but acknowledged that some of Lula's campaign ads had been mistakenly deactivated due to technical errors.
