A far-right libertarian candidate won Argentina’s open presidential primary election on Sunday, a surprising showing for a politician who wants to adopt the U.S. dollar as Argentina’s official currency and embraces comparisons to Donald Trump.

Javier Milei, 52, a congressman, economist and former television pundit, secured 30 percent of the vote with 96 percent of the ballots counted, making him the front-runner for the presidency in the fall general election.

Polls had suggested that Mr. Milei’s support was at about 20 percent, and political analysts had predicted that his radical policy proposals — including abolishing the country’s central bank — would prevent him from attracting many more voters.

But the vote on Sunday made clear that Mr. Milei now has a clear shot at leading Argentina, a South American nation of 46 million with some of the world’s largest reserves of oil, gas and lithium.

“I think these results are surprising even to him,” said Pablo Touzon, an Argentine political consultant. “Up until now, he was a protest candidate.”

Argentina’s general election in October, which could go to a November runoff, will now become a new test of the strength of the far right around the world. Although hard-right forces have gained new influence in several powerful nations in recent years, including the United States, Germany, France, Italy, Sweden and Finland, they have also suffered some defeats, including in Spain and Brazil.

Mr. Milei has pitched himself as the radical change that the collapsing Argentine economy needs, and he could be a shock to the system if elected. Besides his ideas about the currency and the central bank, he has proposed drastically lowering taxes and cutting public spending, including by charging people to use the public health care system; closing or privatizing all state-owned enterprises; and eliminating the health, education and environment ministries.

Sergio Massa, Argentina’s center-left finance minister, finished second in the primary, with 21 percent of the vote. Patricia Bullrich, a conservative former security minister, finished in third place, with 17 percent.

The general election takes place on Oct. 22, but it appears likely that the race will be decided in a runoff vote on Nov. 19. The Sunday results showed that Argentina’s three separate coalitions have similar levels of support, making it unlikely that any candidate will exceed the 50-percent threshold necessary to win outright in the first round.

The center-right coalition’s candidates received a combined 28 percent of the vote on Sunday, while the center-left coalition received 27 percent — both slightly less than Mr. Milei’s total.

The incumbent center-left party has held power in Argentina for 16 of the past 20 years and has been controlled largely by former President Cristina Fernández de Kirchner.

“We’re not only going to end Kirchnerism, but we’re also going to end the useless, parasitic, criminal political caste that is sinking this country,” Mr. Milei told supporters in a speech on Sunday night. He then thanked his sister, who runs his campaign, and his five Mastiff dogs, each named after a conservative economist.

Argentina, which has weathered economic crises for decades, is in the midst of one of its worst. The Argentine peso has plummeted in value, annual inflation has surpassed 115 percent, nearly 40 percent of the population is impoverished and the country is struggling to repay its $44 billion debt to the International Monetary Fund.

Mr. Milei has said that his economic policies would represent an austerity package that goes beyond even what the I.M.F. is requesting of Argentina.

He could also have a profound effect on other parts of Argentine society. He and his running mate, a lawyer who has defended the country’s past military dictatorship, have suggested they would loosen gun laws, reverse recent policies allowing abortion and even permit the sale of human organs, an example of commerce that Mr. Milei says the government has no business restricting.

Yet implementing such changes would lead to a major challenge. Sunday’s results suggested that Mr. Milei, if elected, would have limited direct support in Congress. His party, called Liberty Advances, said it would control just 8 of the 72 seats in the Senate and 35 of the 500 seats in the House, according to the results for its other candidates.

Mr. Touzon said Mr. Milei would have less institutional support than far-right candidates who were swept into office elsewhere in recent years, including Mr. Trump and former President Jair Bolsonaro of Brazil. “Bolsonaro leaned on the army. Trump had the Republican Party. Milei has nothing,” he said.

He added that Mr. Milei’s economic plan, while radical, is lacking in details and has been revised frequently. “His dollarization plan was changed 50 times,” Mr. Touzon said. “Today, he does not have a team to govern Argentina.”

Yet Mr. Milei has proved to be a skilled politician in the internet age, with a trademark scowl and head of unruly hair that have given him a larger-than-life persona and made him an easy subject of internet memes, much like Mr. Trump and Mr. Bolsonaro.

In a public video posted online ahead of the vote, Mr. Bolsonaro endorsed Mr. Milei and said they were political kindred spirits. “We have a lot of things in common,” he said, citing what he called their support for private property, freedom of expression, the free market and the right to self-defense.

And not unlike supporters of Mr. Trump and Mr. Bolsonaro, Argentines who voted for Mr. Milei said on Sunday that they liked him because he was a political outsider who would shake up a broken system and tell it like it is.

“The Argentine people have finally woken up,” said Rebeca Di Iorio, 44, an administrative worker celebrating at Mr. Milei’s election-night street party in Buenos Aires. “Argentina needs that. It needs a change.”

Santiago Manoukian, research chief of Ecolatina, an Argentine economic consulting firm, said that of the different scenarios for primary results that analysts had mapped out, Mr. Milei’s victory was the least expected.

Now Mr. Manoukian said he would have to rethink his predictions of the election, as Mr. Milei has a clear chance to reach the second round, which then could be a tossup.

“He was not seen as a competitive candidate for a runoff,” Mr. Manoukian said. “Now something very different is happening.”