A nationalist former army commander retains a slight lead, and the other two leading candidates are closely tied.
According to Peru's election authority, Ollanta Humala has won 27% of the 53% of the votes counted.
The figure is far below the 50% threshold he would need for outright victory, meaning a runoff with the second-placed candidate next month or in early June is the most probable outcome.
The pro-business conservative candidate, Lourdes Flores, intending to be the country's first female leader, was in second with 25.6%, marginally ahead of the centre-left former president, Alan Garcia.
If he wins outright, Humala, an ally of Hugo Chavez, the Venezuelan president, would be the latest in a series of populist Latin Americans who have risen to power challenging US policies.
"Today, with our vote, we have the chance to initiate a major transformation of our country," Humala said after the elections on Sunday.
If Garcia slips past Flores and into the second round, it would duplicate the candidates' performance in 2001, when polls had shown Flores in second place behind the current president, Alejandro Toledo, until Garcia achieved a late poll surge and beat her into the runoff by 2%.
The prospect of a Humala-Garcia runoff has made many in the business world nervous after Garcia's 1985-1990 presidency left Peru's economy in poor condition.
Voting was largely calm on Sunday. Two small explosions were heard in a coca-growing area in the central Andes while the polls were open, but caused no harm or disruption to the process.
And in the capital, Lima, thousands of people mobbed Humala as he voted in a middle-class neighbourhood, shouting "Murderer, murderer" and "Ollanta is Chavez!"
Reformist Lourdes had hoped to
be Peru's first woman leader
Some, including wealthy women holding designer handbags, hurled rubbish at him before he was escorted away by riot police carrying shields.
Humala has divided Peruvians. Peru's upper classes and business leaders worry that he represents a return to autocratic rule.
But the poor, who feel that they have not benefited from five years of economic growth, have warmed to his promises to redistribute the country's wealth to help them.
Humala has also faced allegations of human-rights abuses as an army commander, which he denies.
"I'm a victim of an anti-democratic campaign, a political ambush," Humala said, referring to the protests.
Viewed as an outsider in a country where the political class is widely discredited, Humala pledges to increase state control of the economy and increase taxes on mining companies with "excessive" profits.
He also has pledged to industrialise the production of coca, the raw material for cocaine, and block a free-trade agreement with the US - raising concern in Washington.