Argentine voters have punished the country’s two main political forces in a primary election, pushing a rock-singing libertarian outsider candidate into first place in a huge shake-up in the race towards presidential elections in October.

With some 90 per cent of ballots counted, far-right libertarian economist Javier Milei had 30.5 per cent of the vote, far higher than predicted, with the main conservative opposition bloc behind on 28 per cent and the ruling Peronist coalition in third place on 27 per cent.

The result is a stinging rebuke to the centre-left Peronist coalition and the main Together for Change conservative opposition bloc with inflation at 116 per cent and a cost-of-living crisis leaving four in 10 people in poverty.

“We are the true opposition,” Mr Milei said in a bullish speech after the results.

“A different Argentina is impossible with the same old things that have always failed.”

Voting in the primaries is obligatory for most adults and each person gets one vote, making it in effect a dress rehearsal for the October 22 general election and giving a clear indication of who is the favourite to win the presidency.

The October election will be key for policy affecting Argentina’s huge farm sector, one of the world’s top exporters of soy, corn and beef, the peso currency and bonds, and ongoing talks over a $US44 billion ($68 billion) debt deal with the International Monetary Fund.

The economic crisis has left many Argentines disillusioned with the main political parties and opened the door for Mr Milei, who struck a chord especially with the young.

“Inflation is killing us and job uncertainty doesn’t let you plan your life,” said Adriana Alonso, a 42-year-old housewife.

As polls closed in the early evening after voting system glitches caused long lines in capital Buenos Aires, all the talk in campaign hubs was about Mr Milei, a brash outsider who has pledged to shutter the central bank and dollarise the economy.

“Milei’s growth is a surprise. This speaks of people’s anger with politics,” said former conservative President Mauricio Macri as he arrived at Together for Change’s election bunker.

In the most important leadership race, within the Together for Change coalition, hard-line conservative Patricia Bullrich, a former security minister, beat out moderate Buenos Aires Mayor Horacio Larreta, who pledged to get behind her campaign.

Economy Minister Sergio Massa won the nomination for the ruling Peronist coalition, as expected, and could perform more strongly in October if he can win over more moderate voters.

The unpredictable factor had been Mr Milei, whose loud rock-style rallies are reminiscent of ex-US president Donald Trump, but he far outperformed all forecasts.

Most polls had given him just shy of one-fifth of the likely vote, though were also badly wrong four years ago in the 2019 primaries.

Turnout was under 70 per cent, the lowest for a primary election since they started to be held in Argentina over a decade ago.

-Reuters