PORT-AU-PRINCE, Haiti – International observers played down allegations of widespread fraud in Haiti's critical post-quake election as ballots began to be counted at an industrial park in the capital Monday.

At the same time, a pair of leading presidential candidates, each sensing a potential berth in a likely runoff, broke with rivals who had said in the midst of voting Sunday that the election was illegitimate and should be thrown out.

The largest observer mission, a joint effort by the Organization of American States and the Caribbean Community, said in a report Monday that the election was marred because many people were blocked from voting by rampant disorganization.

It also cited fraudulent repeat voting facilitated by poll workers and party agents, "deliberate acts of violence and intimidation to derail the electoral process" and other infractions.

But after considering those problems, the mission concluded that it "does not believe that these irregularities, serious as they were, necessarily invalidated the process.

The mission's 120 observers had planned to visit about 40 percent of the 1,500 voting centers, OAS Assistant Secretary-General Albert Ramdin said before the vote. Other observers included a small European Union team, a much larger group of national observers and visitors including a delegation of 12 U.S. Congress members.

Concerns about the potential for unrest have risen since many of the 19 presidential candidates gathered at mid-afternoon Sunday to allege that the voting fraud was designed to help President Rene Preval's preferred successor, state construction company chief Jude Celestin. Demonstrations against the election were held Monday in St. Marc, Gonaives, Carrefour and other cities, but most were quickly dispersed by police.

Haitian-American singer Wyclef Jean, an influential figure in Haiti, warned that violence could erupt if trustworthy vote results are not quickly announced. Flashing his ink-stained thumbnail to show that he had voted, the entertainer called for ballot counting to be concluded immediately — well ahead of the Dec. 7 deadline for preliminary results.

In 24 hours, if we procrastinate, the country will rise to a level of violence that we have never seen before," Jean said.

Anger has boiled since Sunday morning. With the provisional electoral council seen by many Haitians as a tool of the president and distrusted after the unexplained pre-vote disqualification of several parties, voters were incensed when they were unable to find their polling places or their names on electoral rolls. The anger was intensified by the failure of a voter information cell-phone system that was reportedly overloaded by calls.

Violence that killed at least two people, ripped-up ballot boxes and general chaos at polling stations in the capital and elsewhere fueled the perception among many people that the election had been a sham.

By afternoon, 12 of the 19 candidates on the presidential ballot joined forces while polls were still open to charge the election was rigged in favor of Celestin. They stood shoulder to shoulder at a dramatic news conference and called for the vote to be canceled.

But on Monday, two leading members of that group backed away from the call for the election to be voided. Michel "Sweet Micky" Martelly, a popular singer, and Mirlande Manigat, a former first lady, stayed away from a second-day news conference with the other opposition candidates.

Both are widely expected to have a strong chance at facing each other or Celestin in the second round, planned for January if no candidate wins at least 50 percent of the vote. Thousands of people took to the streets Sunday night in Port-au-Prince and Cap-Haitien, most chanting Martelly's name and carrying his picture.

Martelly staged his own widely attended session with reporters Monday. Though he continued alleging fraud and criticizing Preval and Celestin, he denied belonging to any candidate coalition and demurred when asked if he would follow through and boycott the second round.

By evening, Martelly's spokeswoman told The Associated Press that he no longer believed there had been massive fraud and said he agreed with assessments that the election had been a success.

Manigat said on her website that she would not be scheduling media interviews until further notice.

The United States, which spent $14 million on the election, did not immediately have a reaction to the OAS report.

We are in the process of taking a look (and) consulting with our partners in the international community to get a better sense of it," U.S. embassy spokesman Jon Piechowski told AP.

Earlier in the day State Department spokesman P.J. Crowley called the vote "a significant step for Haiti.

This is an election that will determine the government that will lead the reconstruction of Haiti. It's vitally important that this process produce a government that the Haitian people can support," he said.

U.N. Secretary-General Ban Ki-moon said through a spokesman that he was concerned about reports of violence and that he "looks forward to a solution to the political crisis.

Protesters vowed to remain in the streets if Celestin is declared the victor.

Celestin will bring us the same problems as Preval. We will be here every day until the (electoral council) declares (Jean-Henry) Ceant president," said Junior Jean-Pierre, 25. His candidate, another with potential to make the second round, is a lawyer and ally of exiled President Jean-Bertrand Aristide.

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Associated Press writers Ben Fox and Jacob Kushner contributed to this report