The election, already delayed by a political crisis, is also clouded with uncertainty over the return of ousted former President Jean-Bertrand Aristide, a popular but divisive figure whose mere presence was considered by the U.S. government and others as a possible threat to the vote.
Mirlande Manigat, the former first lady, and Michel "Sweet Micky" Martelly, a star of Haitian compas music, emerged as the top two finishers from a first-round, 18-candidate vote in November that was marred by fraud and disorganization.
Lines formed outside polling stations before dawn Sunday as many people sought to cast ballots before going to church. Some polling stations opened several hours late - including the station where Martelly is registered to vote - and workers could be seen setting up after the scheduled 6 a.m. (7 a.m. EDT; 1100 GMT) start time. A spokesman for the electoral council told The Associated Press that poll supplies such as ballots and ink were delayed in reaching voting centers in the southern, southwestern, and western regions of Haiti.
Richardson Dumel, a spokesman for the Provisional Electoral Council overseeing the elections, said the council "is doing its best to send materials."
Edmond Mulet, head of the U.N. mission in Haiti, said there were fewer delays and other problems than in the Nov. 28 first round, which was marred by disorganization and allegations of fraud. "Everything is peaceful, is more or less OK, much better than Nov. 28," Mulet told The Associated Press as he toured polling stations.
Whoever wins will face major challenges, including a Senate and Chamber of Deputies controlled by the party of outgoing President Rene Preval, who was barred by the constitution from running for re-election. They may also face a surge in cholera once the rainy season starts and anger over the fact that 800,000 people are still in what were once optimistically labeled "temporary settlement camps" after the January 2010 earthquake.
"Everybody is waiting for these elections to be done and nobody wants to make a move until they are," said Yves Colon, a Haitian-born journalism professor at the University of Miami. "Haitians are looking for someone who can take them out of this hole they're in."
The two candidates have similar agendas, promising to make education universal in a country where only half the children attend school, to build homes and to foster economic growth. Both have said they want to restore Haiti's armed forces, eliminated by Aristide in 1995 after a long history of abuses.
Their backgrounds could not be more distinct: Manigat is a 70-year-old university administrator and former senator; Martelly is a 50-year-old pop star who has no college degree and a history of crude onstage antics.
Some view his outsider status as an attribute in a country where the government has failed to provide basic services.
"We want to start with somebody who's new, somebody who hasn't been in politics before," said Robenson Naval, a 34-year-old unemployed plumber who lives in a camp across from the ruined National Palace. "We've been trashed by the previous political leaders. They took our votes and dragged them in the ground."
Ebert Cineus, a 28-year-old elementary school teacher, said he was concerned over Martelly's lack of experience.
"Martelly says he will send all children to school for free, but that's an impossible dream," Cineus said. Manigat "is someone who knows how to negotiate. She can get the international community to help this country change."
One of Martelly's most high-profile supporters, hip-hop star Wyclef Jean, was treated at a hospital for a gunshot wound to his hand late Saturday, a spokesman said. The details surrounding the shooting were unclear.
What remains a mystery is what effect, if any, Aristide might have on the race.
The former president, who was ousted in a 2004 rebellion, made a triumphant return from exile Friday - two days before the election, sparking feverish speculation over his motivations and intentions, even though his party was barred from the ballot.
His endorsement, if he offered one, could be a boon for one of the two candidates. If he told followers to boycott the election, it could disrupt the vote and add an influential voice to critics who say it lacks legitimacy.