BRASILIA — Luiz Inacio Lula da Silva won a fourth term as Brazil's president on Sunday, defeating Senator Flavio Bolsonaro in a runoff that the Superior Electoral Court called shortly after 9pm in Brasilia, and that his opponent immediately said he would try to annul.

Lula took 51.76% of valid votes to Bolsonaro's 48.24%, according to the court's count. With all ballots tallied, the court described his lead of nearly 4.4 million votes as unassailable, the Associated Press reported.

The result matters beyond Brazil. Lula had finished second in the first round on 4 October, and no candidate had come from behind to win a runoff since Brazil returned to direct presidential elections in 1989. His victory also denies the Trump-aligned right in Latin America its largest prize, after right-wing candidates won presidential races in Argentina, Peru, Colombia, Bolivia and Ecuador.

Investors had bet the other way. The iShares MSCI Brazil exchange-traded fund fell nearly 20% in overnight trading in the United States after the result, CNBC reported. Bolsonaro had been seen as the likelier of the two to cut spending and deregulate.

Bolsonaro, 45, refused to concede. Speaking to reporters outside his home in Brasilia, he called the result "inexplicable" and said he would seek to have the vote annulled, NPR reported. "I will not give up on my Brazil, and I hope you won't give up either," he said.

His complaint centres on election day itself. About three minutes before polls were due to close, Supreme Court Justice Cristiano Zanin ordered voting extended by two hours, until 7pm local time, after requests from both campaigns. Lula's side had argued that some local governments cut bus services in Sao Paulo and Salvador, making it harder for his working-class voters to reach the polls. Bolsonaro's campaign had asked for more time because heavy rain closed polling stations in the south, where his support is strongest.

The extension did not work evenly. Some polling stations had already switched off their electronic voting machines and could not restart them, NPR and the AP reported. Bolsonaro said the court had "changed the rules of the game mid-game," according to Al Jazeera, and argued that stations which failed to reopen cost him votes. Nikolas Ferreira, a congressman allied with Bolsonaro, pointed out that Zanin is Lula's former lawyer.

Lula, who turns 81 on Tuesday, claimed victory before a crowd in Sao Paulo. He called the election a "defeat of denialism. A defeat of lies," NPR reported. In a line aimed at Washington, he said Brazil would not accept being "anyone's backyard."

The campaign was shaped by the United States. The Trump administration imposed 50% tariffs on some Brazilian goods last year in an effort to halt the prosecution of Flavio Bolsonaro's father, former president Jair Bolsonaro, and sanctioned judges involved in the case, including Justice Alexandre de Moraes, Al Jazeera reported. Jair Bolsonaro is serving a 27-year sentence under house arrest for plotting a coup to overturn his 2022 defeat to Lula, and his son had promised to pardon him if elected.

After the first round, Donald Trump called Bolsonaro's lead "a very, very big victory," according to CNBC. On Sunday the US president said he was watching the vote but declined to endorse either candidate, the AP reported.

Mexico's president, Claudia Sheinbaum, was among the first foreign leaders to react, writing that "the people of Brazil have won," according to Al Jazeera.

The first round had gone the other way, with Bolsonaro on 47.03% to Lula's 45.16%, a gap of about 2.2 million votes. In the three weeks that followed, Lula campaigned on the cost of living, backing a constitutional amendment passed this month that shortened the working week, and argued that a Bolsonaro presidency would leave Brazil under American control. "I think people woke up about the risk of a Bolsonaro government after the first round, and the mobilization made a difference," Alessandra Miguel, a 35-year-old spa worker who voted in Rio de Janeiro's Rocinha favela, told NPR.

Governing will be harder than winning. Bolsonaro's Liberal Party holds the largest bloc in Congress in decades, and Lula acknowledged on Sunday that he would have to work with a legislature he did not pick. "I do not choose the Congress," he said, according to Al Jazeera, adding that "it was the people who chose it."

The public finances are the other constraint. Brazil's gross debt reached 82.9% of GDP in August, its highest level since March 2021, according to the central bank, driven by double-digit interest rates and rigid mandatory spending. Goldman Sachs analysts expect only "modest measures" to slow spending growth, alongside further tax increases, CNBC reported.

Lula first held the presidency from 2003 to 2010, when the economy boomed and millions of Brazilians rose out of poverty. He was jailed for 580 days on corruption convictions that the Supreme Court later annulled, and he returned to office in 2023 after beating Jair Bolsonaro.

Brazilian markets reopen on Monday, when investors expect heavy volatility, CNBC reported. Bolsonaro's promised challenge to the voting extension has yet to be filed, and the court has not formally confirmed the result. Lula is due to be sworn in for a term running to 2030 on 5 January 2027, the oldest president to take office since Brazil's return to democracy.