(Adds comment from winning candidates)

By Helen Murphy and Steven Grattan

BOGOTA, May 27 (Reuters) - Right-winger Ivan Duque, who wants to overhaul a peace deal with Marxist rebels, won Colombia's first-round presidential election on Sunday, setting up a runoff next month with leftist Gustavo Petro, who has pledged to confront privileged elites.

The first election since the peace accord was signed with the Revolutionary Armed Forces of Colombia (FARC) in 2016 now heads for a June 17 runoff to select the successor to President Juan Manuel Santos, who won the Nobel Peace Prize for ending the five-decade-old conflict.

Duque, 41, has pledged to toughen the terms of the peace deal and ensure former rebels pay for their war crimes, while Petro has provoked alarm among investors with pledges to overhaul Colombia's orthodox economic policy and redistribute wealth from the rich to the poor.

With almost all polling stations having reported results, Duque led with 39.1 percent of the vote, as expected, while Petro was a distant second with 25.1 percent.

It was the first time in Colombia's modern history that a leftist candidate has reached the second round of a presidential poll.

Center-left mathematician Sergio Fajardo came a close third, with 23.8 percent, but he declined to endorse either candidate for the second round, saying his voters would make up their own minds.

Campaigning in the traditionally conservative South American nation has been marked by acrimonious accusations that Duque and Petro - from opposite ends of the political spectrum - risked dragging the country back to the battlefield or collapse the economy with socialist policies.

But with Fajardo and the other losing candidates committed to the peace process, Duque may need to soften his stance against the FARC to attract wavering voters, while emphasizing the dangers he says Petro poses for the economy.

Petro, 58, in turn, may seek to allay concerns that his economic policies would lead to a crisis of the kind unfolding across the border in socialist-run Venezuela.

"I'm worried ... Populism is strengthening," said Nicolas Goyeneche, a 28-year-old student, wearing a Duque hat at the candidate's celebration party in the capital Bogota. "But in the second round, we'll beat populism."

Business-friendly Duque was handpicked by hardline former President Alvaro Uribe, who is seen as the power behind his campaign. As well as impose tougher punishments on former FARC fighters, Duque has promised to cut corporate taxes and support oil and mining projects.

"We don't want to shatter the agreement, we want to make it clear that a Colombia of peace is a Colombia where peace meets justice," he said as supporters waved balloons and chanted his name.