Associated Press journalists saw flames leaping from the roof of the Unity party headquarters, the center of Celestin's campaign. Witnesses said the building in central Port-au-Prince was on fire for an hour.
Protesters say security guards shot demonstrators as they assaulted the building, but there were no confirmed injures in the fire or demonstration. Several fire trucks were trying to control the blaze -- an unusual scene in a city with few reliable public services.
The official preliminary results have law professor and former first lady Mirlande Manigat in first with 31.4 percent of the vote and Celestin next with 22.5. Martelly has 21.8 percent -- trailing Celestin by about 6,800 votes.
The results, announced after hours of delays Tuesday evening, were immediately questioned at home and abroad. The U.S. Embassy said the results did not match reports by official election observers who said Celestin would likely be eliminated in the first round.
An appeals period is open for the next three days, and election observers said a third candidate might be included in a Jan. 16 runoff if the electoral council decides the first-round vote was close enough -- though the constitutionality of such a move would be debatable.
But protesters did not wait. Groups of young men armed with clubs and sticks marched through the streets to government buildings, electoral offices and foreign embassies. Several hundred converged on the imposing U.S. Embassy complex on the eastern edge of the capital.
"We want to tell them Martelly is the president. We won't accept anything less. Otherwise we will set this country on fire," Frances Odis said.
Martelly supporters also set up flaming barricades in Petionville, the smoke of burning tires blackening the air. Thousands were on the streets, singing political songs and chanting for "Micky."
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 following the election results.
American Airlines halted 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.
In Les Cayes residents said government buildings had been attacked and set on fire.
Martelly had said before results were released that he believed he would win, and would not accept a spot in a runoff in which Celestin is present. He had not made a statement Wednesday, though some of his staffers had praised the protests on Twitter.
The Nov. 28 election was plagued by allegations of fraud. Thousands of voters were disenfranchised by confusion on the rolls and there were many reported incidents of ballot-stuffing, violence and intimidation confirmed by international observers.
Turnout was low according to the preliminary results, as 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.
Officials acknowledged the rolls were both bloated and incomplete, with hundreds of thousands of earthquake dead still registered and many living voters waiting for ID cards. In the last days of counting, tabulators had to sort out clearly fraudulent tally sheets.
U.N. Secretary-General Ban Ki-moon said the problems were worse than originally reported. But the U.N. peacekeepers and the joint Organization of American States-Caribbean Community observer mission said the problems did not invalidate the vote.
The chief observer with the OAS-Caricom mission, Colin Granderson, appealed for patience.
"Remember that the results are only preliminary results," Granderson told AP. "For all candidates who believe there were irregularities or fraud, there are recourses provided by the electoral law."
Manigat, a 70-year-old law professor, is the wife of former Haitian president Leslie Manigat who served briefly in the late 1980s after a much-criticized election before being deposed by a coup. Her supporters include a powerful senator who organized violent protests in his home department ahead of the first round of voting.
Celestin, a virtual unknown before the election, is the candidate of Preval's Unity party. He is the head of the state-run construction company whose trucks carted bodies and limited amounts of rubble out of the city after the Jan. 12 quake.
His campaign was the best-funded of the group but Preval's inability to jump-start a moribund economy or push forward reconstruction after the massive earthquake drained his support. Many voters said they would accept "anyone but Celestin," whom they equate with the unpopular Preval.
The clear winner in the bid for Senate seats was Preval's Inite, or Unity party, which advanced to a run-off in eight of the 10 races in which it competed and won a ninth outright.
Independent candidate, Steven Benoit, a former member of Preval's abandoned Lespwa movement who championed an increase in the minimum wage, won the 11th Senate seat for the area including Port-au-Prince.