With almost all votes counted, Mr Milei had won close to 56% in the decisive run-off, ahead of his left-wing rival, Sergio Massa, with 44%.

The radical newcomer's victory has been described as "a political earthquake".

It has been welcomed by like-minded politicians such as US ex-President Donald Trump, who said Mr Milei would "Make Argentina Great Again".

Brazil's former leader Jair Bolsonaro said that "hope would shine again in South America".

Sometimes dubbed "El Loco" (the madman) by his critics, Mr Milei has promised drastic changes, which include ditching the local currency, the peso, for the US dollar and "blowing up" the central bank in order to prevent it from printing more money, which he argues is driving inflation.

He has also proposed cutting welfare payments and slashing bureaucracy by closing the ministries of culture, women, health and education, among others. 

In a round of media interviews following his election win, he said he would privatise Argentina's state energy company, YPF, and the country's public broadcasters.

"Everything than can be [put] into the hands of the private sector, will be in the hands of the private sector," he said. However, Mr Milei added that before YPF could be privatised, it would have to "rebuilt". He did not say how long that process could take. 

The president-elect also announced that public works would be "cut down to zero" and those already in progress would be put out to tender so that "there would be no more state spending".

On social issues, he wants to loosen gun laws, abolish abortion - which was legalised in Argentina in 2020 - and allow the sale and purchase of human organs. 

Mr Milei's victory comes amid a deep economic crisis which has seen annual inflation rise to 143% and 40% of Argentines living in poverty.

While opinion polls conducted before the election had given Mr Milei a slight lead over Mr Massa, the wide margin of his win - by more than 11% according to provisional results - has surprised many.