Ten of the 19 presidential candidates marched through Haiti's capital on Sunday to demand officials annul the elections.
"We are here to protest. We want the elections annulled. There were no elections on the 28th," said Charles Baker, one of the presidential candidates denouncing the Nov. 28 election.
Riot police fired tear gas in a bid to disperse the march while some of an estimated 1,500 protesters burned tires on the street and threw stones at UN vehicles.
The presidential hopefuls were part of a group of 12 candidates who called for voting to be cancelled while polls were open, alleging the election had been rigged by President René Preval in favour of the ruling Unity party's candidate, state construction company chief Jude Celestin.
The election was riddled with problems: Polls opened late and many registered voters were improperly turned away from polls.
Observers with the Organization of American States-Caribbean Community also cited instances of violence and voter intimidation, but said the problems were not widespread enough to invalidate the election. Other observers said the vote was tainted.
Confusion appeared to be one major obstacle to the election. Many voters had no idea where to vote, while others arrived at polling stations to find that their names were not on the rolls.
Some said they found their names on one list, outside a voting-room door for instance, only to find that they were not on the list inside. There were also claims of stolen ballot boxes and allegations of ballot-stuffing.
The winner of the presidential race will have to deal with Haiti's crushing poverty, a cholera epidemic that has killed more than 1,900 people and post-earthquake reconstruction, including overseeing billions of promised aid dollars from the United States and other nations.