Ramon Espinosa/Associated Press

Jean-Claude Duvalier, the former dictator known as Baby Doc, read a statement to reporters Friday in Port-au-Prince, Haiti.

A leading opposition candidate, who contends that he qualified for the runoff and not the candidate supported by Mr. Préval, warned that his supporters would take to the streets if he was kept off the ballot.

And the United States, which like a number of nations supports the claims of the opposition candidate, Michel Martelly, sought to press Mr. Préval by announcing that it had withdrawn visas from at least a dozen Haitian officials.

The dispute centers on which candidates finished in the top two positions in the presidential election on Nov. 28. Haiti’s election commission announced preliminary results that showed a former first lady, Mirlande Manigat, finished first and Mr. Préval’s handpicked successor, Jude Célestin, finished second.

But a report released last week by the Organization of American States determined that the election had been rife with fraud and that Mr. Martelly, a popular musician, had actually finished second by a narrow margin, ahead of Mr. Célestin. The report has ignited tensions between Haiti and its most important allies.

In a flurry of meetings with ambassadors from the United States, Canada, France and Brazil, Mr. Préval has stood firm, questioning the methodology of the O.A.S. report and pointing out that the report’s recommendations are not binding.

But binding or not, many of Haiti’s most important international allies have made clear to Mr. Préval that they would withhold recognition of a runoff that is not conducted according to the terms of the O.A.S. report, threatening to block billions of dollars in assistance that had been promised as this desperately poor country struggles to recover from a devastating earthquake a year ago.

Adding to the tensions, the former dictator Jean-Claude Duvalier on Friday made his most extensive public comments since his sudden return from exile last Sunday. Accompanied by three American lawyers, including former Representative Bob Barr of Georgia, Mr. Duvalier said he had returned to Haiti because he wanted to express his condolences to the victims of the earthquake, offer inspiration to Haiti’s youths and encourage national reconciliation.

Looking stronger than he had in previous appearances, he sat alone at a long table while slowly reading a 10-minute statement. He took no questions, waved at a small crowd of cheering supporters and ended his speech with a sweeping apology.

“I want to take this opportunity,” he said, “to express, one more time, my deep sadness to those countrymen who rightly feel that they were victims of my government.”

Mr. Duvalier’s return has sent tremors across an already unsteady political landscape. A political analyst, who asked not to be identified because he was close to the Haitian government and was not authorized to speak publicly, said the United States and France initially accused Mr. Préval of staging the homecoming as a way to distract attention from the political stalemate.

In turn, the analyst said, Mr. Préval accused the United States and France of turning a blind eye to Mr. Duvalier’s movements — he arrived here on an Air France flight from Guadaloupe — to demonstrate how much Haiti’s stability depends on international support.

Describing a meeting between Mr. Préval and several foreign diplomats on the day Mr. Duvalier arrived, the analyst said, “The lack of trust between Préval and the international community was palpable.”

In a statement on Thursday before the United Nations Security Council, the United States ambassador, Susan Rice, said, “Sustained support from the international community requires a credible process that represents the will of the Haitian people, as expressed by their votes.”

In a news conference on Friday, a combative Mr. Martelly accused Mr. Préval of “electoral terrorism,” and vowed that his supporters were “ready to fight for justice.”

Shifting into English, Mr. Martelly said the president’s willingness to risk isolation showed that “Our leaders don’t really care about the future of Haiti.”

When asked whether he saw the United States’ pressure as an infringement on Haiti’s sovereignty, he said, “We have put ourselves in this position because we depend on them for our meals.”

Among the proposals to end the stalemate, a Haitian electoral official said, would be to allow three candidates to compete in the runoff, even though the Constitution prohibits more than two.

Mrs. Manigat said she was also not convinced by the methodology used by the O.A.S. to determine the election results. But she said Mr. Préval should accept the O.A.S.’s recommendation to end the political impasse, restore international relations and prevent political unrest.

“The destiny of Haiti at this moment depends on the will of one man who thinks he can defy the international community,” she said, referring to Mr. Préval.