- Sunday's vote was marked by confusion after a Supreme Court ruling extended voting by two hours as some polling stations had already closed.
Brazil's leftist President Luiz Inacio Lula da Silva pulled off a dramatic comeback on Sunday as he clinched a re-election victory, but his rival Flavio Bolsonaro refused to accept the loss and said he would seek to annul the vote.
With Lula's victory, Brazil staved off a conservative wave that has seen numerous countries -- such as Argentina, Chile, Colombia, and Peru -- all shift right, backed by US President Donald Trump.
The leftist icon, who turns 81 on Tuesday, defied projections to win an unprecedented fourth term.
According to official results, he won 51.8 percent of the votes to 48.2 percent for Flavio Bolsonaro, with Lula pulling around 4.4 million more votes.
The senator is the son of former far-right president Jair Bolsonaro, who is serving a 27-year jail term for attempting a coup after his loss to Lula in the country's last election in 2022.
Sunday's vote was marked by confusion after a Supreme Court ruling extended voting by two hours as some polling stations had already closed.
The move came at the request of Lula who complained that buses had been pulled from circulation in various cities, barring citizens from voting.
Flavio Bolsonaro also requested an extension in some regions due to heavy rains, but he described the decision as "unacceptable" and a "dirty trick"
"It is impossible to recognize the results. I am going to request the annulment of these elections," he said, stone-faced, in a brief speech in Brasilia.
No candidate who lost the first round has gone on to win the second in Brazil since 1989, the first direct election after the end of the country's military dictatorship.
"People went to vote because we managed to convince them that voting was very important for ensuring democracy in the country," Lula said in his first comments after the win.
Fireworks and vuvuzelas rang out in Rio de Janeiro, while thousands of people dressed in red, the color of Lula's Workers' Party, streamed into the streets of Sao Paulo to celebrate.
Lula's fight back
After the first-round defeat, the left swung into action with massive rallies, declaring "Never Again Bolsonaro" and revamped their social media strategy.
The abstention rate decreased from the first round, falling from 21 percent to 19 percent.
Julio Canelo, 42, a lawyer celebrating in Rio de Janeiro hailed a victory for "the entire democratic spectrum."
"It was the result of the people mobilizing, in the streets and online," he told AFP.
A fierce social media battle drew in Meta, parent company of WhatsApp, Facebook and Instagram, which Lula's campaign accused of favoring 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.
"We are going to have to regulate them," Lula said of social media companies after his win.
A bitter campaign marred by scandals and mudslinging laid bare the deep divisions that have plagued Brazil over the past decade.
A country split in two
Lula heads into a fourth term with almost half of the nation having rejected him, desperate for change.
He rose from poverty to become president in 2003 and was re-elected in 2006.
Lula made a dramatic return to office in 2022 elections after a corruption scandal saw him jailed for over 18 months.
The charges were annulled on procedural grounds.
In his third term, Lula oversaw a drop in inflation and unemployment, and lifted millions out of hunger, but many voters felt gloomy about the high cost of living and crime.
Flavio Bolsonaro, 45, pitched himself as more moderate and jovial than his father and managed to galvanize conservatives as well as less ideological voters wanting something new.
"We're in a deep hole, things are dire. People can barely afford to eat -- buying meat is out of the question," said Tereza Cristina Ramos, a 68-year-old retiree, who voted in Rio de Janeiro.
Already one of the world's oldest leaders, Lula will be 85 by the end of his new term in 2030.
He is also facing a hostile Congress dominated by the right.
Lula's campaign leaned heavily on the theme of sovereignty in the face of commercial and diplomatic pressure from Trump's United States.
"This country will not accept being anyone's backyard," he said Sunday.
Lula received congratulations from Chile's far-right president, Jose Antonio Kast, and Ecuador's right-wing Daniel Noboa -- both close Trump allies.
Mexico's left-wing President Claudia Sheinbaum wrote on X, saying "Long live democracy, the people of Brazil won."