At least one person was reported dead, and witnesses said the man, who was wearing a T shirt bearing a picture of Preval, was shot by Jordanian U.N. peacekeepers in the impoverished nation's capital Port-au-Prince.
But U.N. spokesman David Wimhurst denied the account, saying peacekeepers had only fired two warning shots into the air, reported the Associated Press.
Hundreds of demonstrators, meanwhile, stormed a hotel outside Port-au-Prince where election officials had been briefing journalists on the disputed vote.
Barricades made of tires set ablaze were scattered throughout the city, and protesters allowed only journalists and Red Cross vehicles to pass.
"If they don't give us the final results, we're going to burn this country down," a man at one of the roadblocks yelled, according to the AP.
With 90 percent of the votes from Tuesday's election tallied, the Provisional Electoral Council said Preval had 48.7 percent. If a candidate receives less than 50 percent of the vote, a runoff is required on March 19.
When initial results were announced several days ago, Preval had 61 percent of the vote.
Preliminary results show another ex-president Leslie Manigat with 11.84 percent and businessman Charles Baker with 7.9 percent.
Preval is viewed as a champion of the poor but he is distrusted by the wealthy elite. He is a protege of Jean-Bertrand Aristide, who was ousted in a bloody coup in February 2004.