Port-au-Prince, Haiti (CNN) -- Riot police fired tear gas and sent hundreds of demonstrators running a day after Haiti's national elections ended in turmoil amid allegations of widespread fraud.
The incident took place as one of the 18 presidential candidates was denouncing the outgoing president and his endorsed candidate in a news conference broadcast live on Haitian radio.
"I have a message for [Rene] Preval and his acolyte," said Michel "Sweet Mickey" Martelly. "Haiti doesn't want you any more."
As he spoke, hundreds of people, chanting Martelly's name, jogged toward the center of Port-au-Prince until they were intercepted by riot police.
The tide turned when police fired several canisters of tear gas into the crowd, sending Martelly's supporters sprinting for cover.
On Sunday, hours before the polls closed, Martelly and 11 other candidates for president made a joint declaration denouncing the election and calling for its cancellation. The candidates accused Preval of organizing widespread fraud in favor of his Unity Party, which is fronted by establishment candidate Jude Celestin.
The dissenting candidates did not offer evidence to back up their claims.
Senior officials from Haiti's election organizing committee, the Conseil Electoral Provisoire, defended the vote and calling it a success.
"The election day has successfully concluded," said the committee's director general, Pierre Opont, in a news conference Sunday night. Opont said there had been incidents at 56 polling centers out of a total of 1,500 across the country, less than 4 percent. He also said at least one person died as a result of election violence.
However, there were widespread reports Sunday of disorganization and some cases of apparent cheating at the polls.
"We had observers in about 50 polling stations, and in each one we interviewed countless individuals who could not vote at that polling station because their names weren't on the electoral lists," said Nicole Phillips, staff attorney with the non-profit organization Institute for Justice and Democracy in Haiti.
"If we go ahead and certify the elections," Phillips said, referring to international donors that have helped fund the vote, "then we look bad. We paid for these elections. We organized them. We trained the people. And the fact that it was horrible yesterday looks bad on us."
On Sunday, the United Nations issued a written statement expressing "deep concern at the numerous incidents that marred the elections."
A spokesman for the U.S. Embassy in Port-au-Prince said American diplomats were appealing for calm and waiting for the result of a report expected to be issued by a team of international election observers in a joint delegation from the Organization of American States and the Caribbean Community.
Ambassador Colin Granderson, head of the monitoring delegation, Monday listed a large number of irregularities and problems with the election. However, he said, "the joint mission does not believe that these irregularities, serious as some of them were, necessarily invalidate the process."
He called the decision of the 12 presidential candidates to call for cancellation of elections hours before polls closed "precipitous" and "regrettable."
He said the vote-counting process should continue, and made a fresh appeal for calm among all parties.
In addition to the presidential election, seats also were being contested in the Senate and in the lower house of parliament.
Nearly all of the candidates campaigned on similar platforms -- fighting corruption, creating jobs, and addressing a series of natural disasters that has left the Caribbean nation reeling and prompted many to urge for a postponement of Sunday's vote.
According to electoral law, a candidate must win 50 percent of the vote or the election goes to a second-round runoff.
Haiti's growing political crisis comes as it struggles to cope with a deadly cholera epidemic that has killed more than 1,600 people since last month. On Monday, the turmoil appeared to be spilling over the border. A United Nations Police spokesman confirmed reports of a clash erupting on Haiti's border with the Dominican Republic between soldiers of that nation and Haitian civilians.
"Shots were fired," said spokesman Jean-Francois Vezina. "Our UNPOL people are there and trying to intervene."
Vezina said he could not confirm reports broadcast on Haitian radio that a woman and a girl were killed in the border clash.