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

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

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 $44 billion debt deal with the International Monetary Fund.

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 Milei, a brash outsider who has pledged to shutter the central bank and dollarize the economy.

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.

Whoever wins in October, or more likely in a November runoff, will have big decisions to make on rebuilding depleted foreign reserves, boosting grains exports, reining in inflation and on how to unwind a thicket of currency controls.

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