Argentina has elected libertarian outsider Javier Milei as its new president, rolling the dice on an outsider with radical views to fix an economy battered by triple-digit inflation, a looming recession and rising poverty.
Milei, who rode a wave of voter anger with the political mainstream, won by a wider-than-expected margin on Sunday. He landed some 56 per cent of the vote versus just over 44 per cent for his rival, Peronist Economy Minister Sergio Massa, who conceded.
"The model of decadence has come to an end, there's no going back," Milei said in a defiant speech after the result, while also acknowledging the challenges that face him.
"We have monumental problems ahead: inflation, lack of work, and poverty," he said. "The situation is critical and there is no place for tepid half-measures."
In downtown Buenos Aires hundreds of Milei supporters honked horns and chanted his popular refrain against the political elite - "out with all of them" - as rock music played from speakers. Some people set off fireworks as excitement spread.
Milei is pledging economic shock therapy. His plans include shutting the central bank, ditching the peso, and slashing spending, potentially painful reforms that resonated with voters angry at the economic malaise.
Milei's challenges are enormous. He will have to deal with the empty coffers of the government and central bank, a creaking $US44 billion ($A68 billion) debt program with the International Monetary Fund, inflation nearing 150 per cent and a dizzying array of capital controls.
Some Argentines had characterised the vote as a choice of the "lesser evil": fear of Milei's painful economic medicine versus anger at Massa and his Peronist party for an economic crisis that has left Argentina deeply in debt and unable to tap global credit markets.
Milei has been particularly popular among the young, who have grown up seeing their country lurch from one crisis to another.
Milei's win shakes up Argentina's political landscape and economic roadmap, and could impact trade in grains, lithium and hydrocarbons. Milei has criticised China and Brazil, saying he won't deal with "communists," and favours stronger US ties.
Despite that, Brazilian President Luiz Inacio Lula da Silva wished Milei luck and success after the result was announced, adding that it was important democracy was respected.
Former US president Donald Trump congratulated Milei and said the libertarian would make Argentina great again.
Leftist Colombian President Gustavo Petro, meanwhile, said it was a "sad day" for the region.
The victory of Milei, a 53-year-old economist and former TV pundit, has broken the hegemony of the two leading political forces on the left and right - the Peronists that have dominated Argentine politics since the 1940s and its main opposition, the Together for Change conservative bloc.
"The election marks a profound rupture in the system of political representation in Argentina," said Julio Burdman, director of the consultancy Observatorio Electoral, ahead of the vote.
The campaign of Massa, 51, an experienced political wheeler-dealer, had sought to appeal to voter fears about Milei's volatile character and plans to cut back the size of the state.
Milei is staunchly anti-abortion, favours looser gun laws and has criticised Argentine Pope Francis. He used to carry a chainsaw in a symbol of his planned cuts but shelved it in recent weeks to help boost his moderate image.
After October's first-round vote, Milei struck an uneasy alliance with the conservatives. But he faces a highly fragmented Congress, with no single bloc having a majority, meaning that he will need to get backing from other factions to push through legislation. Milei's coalition also does not have any regional governors or mayors.
That may temper some of his more radical proposals. Long-suffering voters are likely to have little patience, and the threat of social unrest is never far below the surface.
His backers say only he can uproot the political status quo and economic malaise that has dogged South America's second-largest economy for years.