Brazilian president-elect Lula da Silva delivers a speech to supporters in Sao Paulo, Brazil. AFP

The victory marks a stunning turnaround for the charismatic but tarnished leftist heavyweight, who left office in 2010 as the most popular president in Brazilian history, fell into disgrace when he was imprisoned for 18 months on since-quashed corruption charges, and now returns for an unprecedented third term at age 77.

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All eyes will now be on how Bolsonaro and his supporters react to the result after months of alleging — without evidence — that Brazil's electronic voting system is plagued by fraud and that the courts, media and other institutions had conspired against his far-right movement.

"This country needs peace and unity," Lula said to loud cheers in a victory speech in Sao Paulo.

Lula da Silva supporters celebrate in Brasilia. AFP

"The challenge is immense," he said of the job ahead, citing a hunger crisis, the economy, bitter political division and deforestation in the Amazon.

He later addressed a tightly packed crowd of hundreds of thousands of supporters clad in Workers' Party red who flooded the city center, vowing: "Democracy is back."

'He hasn't called yet'

Bolsonaro, 67, was silent in the hours after the result was declared.

"Anywhere in the world, the losing president would already have called to admit defeat. He hasn't called yet, I don't know if he will call and concede," Lula told the massive crowd.

Some Bolsonaro supporters, gathered in the capital Brasilia, refused to accept the results.

Jair Bolsonaro attends a meeting. File photo

"The Brazilian people aren't going to swallow a faked election and hand our nation over to a thief," said 50-year-old teacher Ruth da Silva Barbosa.

Electoral officials declared the election for Lula, who had 50.9 per cent of the vote to 49.1 per cent for Bolsonaro with more than 99.9 per cent of polling stations reporting, in the closest race since Brazil returned to democracy after its 1964-1985 dictatorship.

Bolsonaro, the vitriolic hardline conservative dubbed the "Tropical Trump," becomes the first incumbent president not to win re-election in the post-dictatorship era.

With no word from Bolsonaro, some of his key allies appeared in public to accept the results. They included the speaker of the lower house of Congress, Arthur Lira, who said it was time to "extend a hand to our adversaries, debate, build bridges."

Agence France-Presse