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

The fallout from the Nov. 28 election, riddled by fraud, is violently shutting down cities across the impoverished country with gunfire and barricades at a moment when medical aid workers need to tackle a surging cholera epidemic that has claimed more than 2,000 lives.

Haiti's Radio Kiskeya said in an unconfirmed report that at least four demonstrators were killed _ three in Les Cayes, about 120 miles (193 kilometers) west of Port-au-Prince in the country's southern peninsula, and one in the northern city of Cap-Haitien.

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.

On Wednesday, demonstrators 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 wearing their shirts as masks threw rocks at U.N. troops. The soldiers _ Indians and Pakistanis working as a single 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 in tow.

Protesters set fire to the headquarters of Preval and Celestin's Unity party. Multiple fire trucks responded to the scene as flames licked the roof _ an unusual scene in a country with few public services _ but in late afternoon piles of charred campaign posters continued to smolder.

"We want Martelly. The whole world wants Martelly," said James Becimus, a 32-year-old protester near the U.S. Embassy. "Today we set fires, tomorrow we bring weapons."

Other protesters said they would continue to mobilize but do so nonviolently, as Martelly urged in a radio address Wednesday afternoon. He also told supporters to watch out for "infiltrators" who might try to incite violence.

"Demonstrating without violence is the right of the people," he said. "I will be with you until the bald-head victory."

Martelly's campaign manager later said the results were "massive fraud" and they would formally protest them to the Provisional Electoral Council.

Preval had earlier urged the candidates to call off the protests. He acknowledged there had been fraud in the election, but said it was typical of elections around the world.

"This is not how the country is supposed to work," he told demonstrators in a live radio speech. "People are suffering because of all this damage."

He also shot back at the U.S. Embassy's reproach, saying, "The American embassy is not the (electoral council)."

Preval's administration has been condemned by many Haitians for failing to spearhead reconstruction of the country after the earthquake. More than an estimated 1 million people still live under tarps and tents and little of the promised international aid from the United States and other countries has arrived.

Preliminary election results put Celestin ahead of Martelly by just 6,845 votes for second place. Former first lady and law professor Mirlande Manigat took first place with 31.4 percent of the vote, while Celestin had 22.5 percent and Martelly 21.8 percent. The top two candidates advance to the Jan. 16 second round.

Manigat also told Haitian radio that she felt her reported vote tally was low. Celestin's managers said before the election that they had expected both a first-round victory and to be accused of fraud.

Thousands were disenfranchised by confusion on the rolls, which were overstuffed with earthquake dead but lacked many living voters. There were reported incidents of ballot-stuffing, violence and intimidation confirmed by international observers, but U.N. peacekeepers and the joint Organization of American States-Caribbean Community observer mission said the problems did not invalidate the vote.

Turnout was low. Just over 1 million people cast accepted ballots out of some 4.7 million registered voters. It is not known how many ballots were thrown out for fraud.

In a televised address, Preval took a swipe at Washington's criticism of the election results, saying that while he was open to discussing electoral problems with anyone, "the American Embassy is not (the electoral council)."

State Department spokesman P.J. Crowley said the U.S. is not fomenting the unrest.

"The United States is in no way responsible for the actions of any individual. What we are determined to help Haiti achieve is a credible election and a result _ not one that the United States will impose _ but one that the people of Haiti can participate in fully," he told reporters in Washington.

Martelly had joined with 11 other candidates, including Manigat, to accuse Preval of trying to steal the election while polls were still open.

An appeals period is open for the next three days, and election observers said a third candidate might be included in the runoff if the electoral council decides the first-round vote was close enough _ though the constitutionality of such a move would be debatable.

U.N. Secretary-General Ban Ki-moon expressed concern "about allegations of fraud" and "the acts of violence that have taken place in the aftermath of the announcement," U.N. spokesman Martin Nesirky said at U.N. headquarters in New York.

He said all candidates have a responsibility to encourage their supporters to refrain from violence.

Vehicles were damaged by rocks and items were reportedly stolen from stores. Foreign aid workers complained that Haitian national police were slow to respond and that many officers refused to report to duty.

American Airlines canceled all flights in and out of the Haitian capital because airport employees were unable to get to work Wednesday because of demonstrations, spokeswoman Martha Pantin said. Flights will also be canceled on Thursday.

The U.S. Embassy reported that the smaller regional airport at Cap-Haitien was also closed due to demonstrations and barricaded roads.

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Associated Press writers Jacob Kushner in Port-au-Prince, Ben Fox in San Juan, Puerto Rico, Bob Burns in Washington and Edith Lederer at the United Nations contributed to this report.