PORT-AU-PRINCE, Haiti — Musician Michel “Sweet Micky’’ Martelly scored a come-from-behind victory yesterday in Haiti’s presidential runoff, according to preliminary results from last month’s election showing he easily defeated a former first lady.

Martelly, who has never held political office, will lead a country facing enormous challenges as it tries to recover from last year’s earthquake.

He received nearly 68 percent of the vote in the two-way race with Mirlande Manigat, electoral council spokesman Pierre Thibault said.

Thousands of Martelly supporters poured into the streets of Port-au-Prince, carrying Martelly posters, climbing onto cars, and cheering loudly. A huge crowd of singing and chanting supporters marched to his house.

“Today is a big day for me,’’ Jeanor Destine, 22, said as he ran through the streets. “We’re finished with the old government and want to bring in a new government. We’ve been through so much misery. That’s why we’re supporting Martelly.’’

The musician had trailed Manigat in the crowded first-round election in November. But his campaign gained momentum in the second round, with many voters seemingly enchanted with his lack of political experience.

In a message posted in Creole on Twitter, Martelly said: “Thank you for your confidence. . . . We’re going to work for all Haitians. Together we can.’’

Haiti’s electoral council said that about 23 percent of the 4.7 million registered voters cast ballots. Serge Audate, an elections official, said about 15 percent of the tally sheets had problems suggesting possible fraud, including cases in which more votes were cast than there are registered voters in some polling stations. The suspect sheets were quarantined.

Final results are to be announced April 16.

Yesterday, Haitians in Massachusetts celebrated Martelly’s win. Late last year, he campaigned at a teachers union in Boston and later won a mock election organized by Haitian expatriates, who do not have the right to vote in their homeland.

“He’s a person with his heart and his head in the right place,’’ Elda James, a Dorchester lawyer and Martelly supporter, said yesterday. “He’ll make a positive difference in Haiti.’’

James, whose family fled the Duvalier dictatorship in the 1960s, said Martelly is often portrayed as a bad boy musician. But she said she has known him for 20 years and that he is a serious person who will lead the struggling nation.

Martelly’s campaign for president seemed at first like an afterthought, overshadowed by the short-lived campaign of the better-known star Wyclef Jean, who was declared inelgible to run.

Many said that Martelly’s history of onstage antics would prevent him from winning. Indeed, Manigat, a university administrator and former senator, and her supporters made much of it during the campaign by stressing her morality and urging people to call her “mother.’’

But the 50-year-old Martelly turned out to be a serious and skilled candidate. When initial results of the flawed first round showed he was out of the race, he mobilized supporters to protest as if he were a veteran of Haiti’s rough politics. He ran a disciplined campaign, deftly depicting himself as an outsider and neophyte even though he has long been active in politics.

He promised profound change for Haiti, vowing to provide free education in a country where more than half the children can’t afford school, and to create economic opportunity amid almost universal unemployment.

The candidates were vying to replace President Rene Preval, who was barred by the constitution from serving a third term.

The new president will face a challenging environment that includes a Senate and Chamber of Deputies controlled by Preval’s party and widespread anger over the slow progress of reconstruction from the January 2010 earthquake. Haiti also is grappling with a cholera outbreak that has killed more than 4,000 people since October and is expected to worsen with the spring rainy season.

Maria Sacchetti of the Globe staff contributed to this report.

© Copyright 2011 Globe Newspaper Company.