PORT-AU-PRINCE, Haiti - Protesters enraged by the results of last month's presidential election set barricades and political offices ablaze, traded blows with U.N. peacekeepers and shut the country's lone international airport yesterday, creating the social upheaval many have feared since the Jan. 12 earthquake.

The fallout from the Nov. 28 election, riddled by fraud, is closing down cities across the impoverished country with gunfire and barricades while medical-aid workers are needed to tackle a cholera epidemic that has killed more than 2,000 people.

Haiti's Radio Metropole reported that at least one demonstrator was killed in Les Cayes, about 120 miles west of Port-au-Prince.

The protesters back a popular carnival singer who narrowly lost a spot in a runoff election to Jude Celestin, a political unknown viewed by supporters and detractors alike as a continuation of unpopular President Rene Preval's administration.

The U.S. Embassy criticized the preliminary results Tuesday, saying Haitian, U.S. and other international monitors had predicted that Celestin was likely to be eliminated in the first round.

Demonstrators yestersday carried pink signs with the smiling face and bald head of their candidate, Michel "Sweet Micky" Martelly. They decorated barricades with empty ballot boxes, used government campaign posters to start fires and challenged heavily armored foreign soldiers to near-theatrical confrontations.

Outside the provisional electoral-council headquarters, a former gym in the suburb of Petionville, young men hiding their faces with their shirts threw rocks at U.N. troops.

The soldiers - Indians and Pakistanis working as one unit - responded with exploding canisters of tear gas that washed over a nearby earthquake-refugee camp, sending mothers running from their tarps with their crying, coughing children.

Protesters set fire to the headquarters of Preval and Celestin's Unity party.

As flames licked the roof, multiple firetrucks responded to the scene - an unusual occurrence in a country with few public services - but in late afternoon, piles of charred campaign posters continued to smolder.

Other protesters said they would continue to mobilize but would do so nonviolently, as Martelly urged in a radio address yesterday afternoon.

He also told supporters to watch out for "infiltrators" who might try to incite violence.