Sao Paulo: Brazilian right-wing Senator Flavio Bolsonaro has followed his imprisoned father's lead by refusing to concede defeat to leftist President Luiz Inacio Lula da Silva, who won a historic fourth term in Sunday's presidential election.

Bolsonaro, the son of former President Jair Bolsonaro, called into question a late-Sunday decision by a Lula-appointed Supreme Court justice that extended voting by two hours over alleged voter transport irregularities. He said he would ask for the result to be annulled.

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"It is impossible to recognize the election results, given the circumstances under which things unfolded," Bolsonaro told journalists.

Yet the scale of the loss, with Bolsonaro trailing Lula by over 4 million votes, makes it unlikely his efforts to annul the vote will succeed, analysts said. His petition is also undermined by the fact that his campaign also sought a voting extension due to heavy rains in the conservative-leaning south.

Still, Bolsonaro's refusal to concede could spark a wave of protests in a country where faith in the electoral system, and in judicial independence, has been strained in recent years, observers say.

Jair Bolsonaro plunged Brazil into a deep democratic crisis after the election in 2022, when his supporters stormed government buildings in protest against the result. He later received a 27-year sentence for plotting a coup.

Lula's victory defied polls, precedent and global anti-incumbent sentiment to put the brakes on a wave of right-wing election victories across Latin America. The result will disappoint the administration of US President Donald Trump, whose State Department led a campaign to return the Bolsonaros to power.

Financial markets also expressed initial concerns at the shock result. A Tokyo-listed exchange-traded fund tracking Brazil's Bovespa index dived nearly 15% in Asia trade on news of Lula's victory, while analysts expected the Brazilian real to fall against the US dollar.

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Lula had trailed Bolsonaro in the first round, but managed to convince voters to come out for him in the runoff.

Speaking at a hotel in Sao Paulo late on Sunday, he took a conciliatory tone.

"I won't speak of national unity, because that doesn't always fit the reality of this country. No one is obliged to like the elected president, but he was put in place to govern for everyone," Lula said.

During the campaign, Lula pointed to his track record of cutting poverty and inequality while also highlighting record low unemployment, illiteracy and deforestation, but his fourth term may prove challenging.

He will have to work alongside a Congress packed full of Bolsonaro allies after a surge of right-wing voting recast Brazil's legislature in favor of the president's enemies.

"The challenges are countless," said Carlos Melo, a political science professor at Insper in Sao Paulo.