Libertarian outsider Javier Milei pulled off a massive upset in Argentina's presidential election Sunday, winning around 56 percent of the vote, provisional results showed.

His rival, Economy Minister Sergio Massa, who earlier conceded defeat in a speech, had around 44 percent of the total, with almost 90 percent of votes counted.

"Obviously the results are not what we expected. I have contacted Javier Milei to congratulate him," Massa said in the speech. "From tomorrow the responsibility of providing certainty belongs to Milei."

Milei, who is popular among young voters, 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.

However, Milei's challenges are enormous, as he will have to deal with the empty coffers of the government and central bank, a creaking $44 billion debt program with the International Monetary Fund, inflation nearing 150 percent and a dizzying array of capital controls.

The shock rise of the 53-year-old economist and former TV pundit has been the story of the election, breaking the hegemony of the two main political forces on the left and the right - the Peronists and the main Together for Change conservative bloc.

Milei's win shakes up Argentina's political landscape and economic roadmap, and could impact trade in grains, lithium and hydrocarbons. He is also staunchly anti-abortion, favors looser gun laws and has criticized 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, which boosted his support. 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.

(With input from reuters)

(Cover: Javier Milei waves hand during a presidential election rally in Buenos Aires, Argentina, October 22, 2023. /CFP)