Peru has registered the most robust economic growth in South America this past decade, but on Sunday a leftist former army officer who once pledged to overturn the old economic order is expected to win the first round of presidential elections, polls show.

The rise of the ultranationalist Ollanta Humala, in a country hailed by Wall Street for its economic expansion and market-friendly policies, is a sharp contrast with much of the continent. Politically moderate presidents — some slightly to the left, others slightly to the right — have produced strong economic growth from Chile to Uruguay and Brazil, and their residents have rewarded them with high approval ratings.

In Peru, the economy has grown 6.3 percent since 2002, among the highest growth rates in the world, as U.S. and other foreign companies invested an average of $3 billion each year. Poverty fell 15 percentage points, and signs of affluence can be seen even in the country’s traditionally poverty-stricken southern highlands.

And yet, the country of 30 million is on the verge of electing a man who once said that Peru’s “economic model is finished.”

“It’s an extraordinary anomaly,” said Cynthia McClintock, a Peru scholar at George Washington University, speaking by phone from Lima, the capital, where she went to observe the election. “The economy has been phenomenal and consistent, so why?”

Humala, 48, a former lieutenant colonel who in the past pledged to crack down on the multinationals and expressed kinship with Venezuelan President Hugo Chavez, has the support of about 30 percent of the electorate, polls show. That is not enough to win outright Sunday, but if his support holds, he will make the June 5 run-off.

His likely opponent would be another candidate who sends shivers down the backs of Peru’s traditional politicians and many middle-class voters, Keiko Fujimori. She is the daughter of a former president, Alberto Fujimori, who is serving a 25-year prison term for rights abuses during his decade-long rule in the 1990s.

Peru’s Nobel laureate author, Mario Vargas Llosa, recently said Humala and Keiko in the final round would be a “catastrophe” for Peru.

“I don’t want us to return to the dark past,” added Vargas Llosa, who ran against the elder Fujimori and lost in the 1990 election.

Political analysts said Humala and Keiko Fujimori have mined the discontent of millions who struggle to eke out a living in Peru’s large informal economy. Peruvians have been told their country is flying high, yet wages barely keep up with inflation. Many are appalled by rampant corruption and incensed by rising violent crime.

“People do not feel represented by their politicians,” Carlos Basombrio, a political analyst and government official, said in a phone interview. “We have a supremely inefficient state, you cannot expect reforms, public schools are an embarrassment, and the justice system is a farce.”

Basombrio said it does not help that the current president, Alan Garcia, has hailed Peru as the continent’s rising star when a third of his countrymen live in poverty. “Ordinary people take a look at their lives and, even if they have risen out of poverty, life is precarious and they feel offended,” Basombrio said.

Garcia, to be sure, has paid the price: His approval ratings hover under 30 percent and his party did not even field a candidate.

Humala and Keiko Fujimori have also benefited from a splintering of politically moderate urban voters, who have three candidates from whom to choose, among them a former president, Alejandro Toledo, and his former economy minister, Pedro Pablo Kuczynski.

To attract voters beyond the far left, the compact, wiry Humala, whose name means “warrior who sees all,” has toned down the fiery rhetoric from his first run at the presidency, in 2006.

He has traded the red T-shirt reading “Love for Peru” for a tailored suit. And he does not talk about using his belt to beat corrupt politicians.

He has publicly distanced himself from Chavez, who Humala pledged would become a close ally in the 2006 campaign. Instead, he has compared himself to Luiz Inacio Lula da Silva, Brazil’s popular former president.

Humala’s parents and siblings, who became a liability in the past, have also been kept out of sight. His father, Isaac, had led an ultranationalist sect that held that Indians were racially superior. A brother, Antauro, is serving a jail term for leading an uprising against a police station in 2005 in a bid to overthrow the Toledo government. Humala’s mother once said gays should be shot.

“We don’t believe in violence, we believe in peace,” Humala said Thursday, in his closing campaign speech.

Still, he stressed that his government would mean change, particularly for the big companies and political establishment he accuses of accumulating great wealth at the expense of ordinary Peruvians.

“The government and traditional politicians think it belongs to a small group of multinationals,” he said of the country’s riches. “That it belongs to a few companies, and not to the Peruvian people.”