AMNA NAWAZ: In a closely watched election, Brazil's president, known as Lula, defeated right-wing and Trump-aligned challenger Flavio Bolsonaro last night by nearly 4.5 million votes.

Transportation and weather disruptions in parts of Brazil led to a chaotic final stretch at the polls.

Bolsonaro is now refusing to concede.

And, today, President Trump said the outcome was -- quote -- "rigged."

Stephanie Sy reports.

STEPHANIE SY: On the streets of Rio de Janeiro last night, rejoicing and cheers of relief rang out among Lula supporters.

Brazilian voters narrowly reelected the leftist president, Luiz Inacio da Silva, the 80-year-old Lula beating right-wing senator Flavio Bolsonaro 51 to 48 percent.

Lula addressed supporters in Sao Paulo after the win.

LUIZ INACIO LULA DA SILVA, Brazilian President (through translator): I want you to know that everything I do, I do with you in mind.

All public policies are designed with you in mind.

And you have given me the distinction of being the first president of the republic to be elected four times through democratic means.

This is a historic achievement that I owe to the Brazilian people.

STEPHANIE SY: It's historic in more ways than one.

Lula was the underdog after he lost the first round of voting earlier this month to Flavio, the son of former President Jair Bolsonaro, who was convicted for attempting to stage a coup.

Millions of Brazilians who did not vote in the first round came out Sunday, with voters in Rio de Janeiro and the suburbs of Sao Paulo swinging their support to Lula.

ANA CANDIDA COTRIM, Lula Supporter (through translator): The feeling is that democracy resisted in the midst of authoritarianism, in the midst of the whole world, of the great powers like the United States against Brazil.

And we managed to resist with the strength of the people in the streets and on social media.

STEPHANIE SY: Lula's victory broke a recent right-wing winning streak in Latin America.

Now Brazil and Mexico, Latin America's two most populous countries, have bucked the trend.

Mexico's President Claudia Sheinbaum reacted jubilantly today.

CLAUDIA SHEINBAUM, Mexican President (through translator): It shows that, when the people decide to mobilize and defend their rights, they always win.

And that is a lesson for Brazil, for Mexico, and for the whole world.

STEPHANIE SY: Lula's win was a loss for President Trump's influence in the region.

And echoing Trump and his father, Flavio Bolsonaro refused to admit defeat last night, saying he planned to request an annulment of the election.

FLAVIO BOLSONARO, Brazilian Presidential Candidate (through translator): What happened today was not normal.

It was downright unacceptable.

A Supreme Court justice for the first time in history, a former lawyer for Lula, with a simple stroke of the pen, at the request of a former partner, changed the rules of the game in the middle of the match.

STEPHANIE SY: Stormy weather and transportation troubles had led both candidates to request an extension of voting by two hours.

But in the end, the country's Supreme Court granted Lula's request shortly before polls were set to close.

Supporters of Bolsonaro are crying foul.

JEREMIAS OLIVEIRA, Brasilia, Brazil Resident (through translator): Flavio Bolsonaro has already said he will file to annul this election.

I hope he manages to, because it's not normal, people.

This is wrong.

STEPHANIE SY: Despite attempts to inject doubt, the results are official.

And even right-wing leaders in the region are congratulating Lula.

Flavio Bolsonaro will remain in the Senate through the end of his term, however, and with Congress still dominated by Brazil's right-wing.

Lula's win, while decisive, is far from a mandate.

For the "PBS News Hour," I'm Stephanie Sy.