Scuffles broke out and polling stations opened hours late Tuesday as masses of Haitians waited — sometimes in mile-long lines — to vote under the protection of U.N. peacekeepers crouching behind machine guns and patrolling alongside armored vehicles.

Outside the gang-controlled Cite Soleil slum, frustrated voters pounded on empty ballot boxes and chanted, "It's time for Cite Soleil to vote!"

Rene Preval, a 63-year-old former president backed by many poor Haitians, is the front-runner, according to pre-election polls. In an interview with The Associated Press, he said "people are investing everything in this election."

Among 33 other presidential candidates are a factory owner whose slogan is "Order, Discipline, Work," and another former president ousted in a coup.

Turnout for the vote — called a key step toward steering this bloodied, impoverished nation away from collapse — all but overwhelmed electoral officials. At dawn, when the 800 polling stations were supposed to open, it immediately became apparent the day would not go smoothly. In the upscale Petionville suburb of the capital, members of a crowd of thousands of voters stormed a voting station. Several women fainted.

Polls closed by late Tuesday — nearly four hours later than scheduled — said Stephane Lacroix, a spokesman for Haiti's elections commission.

"The people have voted massively," said Juan Gabriel Valdes, a U.N. special envoy.

Government officials sought to maintain calm, assuring Haitians that everyone would have a chance to vote. By mid-afternoon, the process appeared more orderly. U.N. troops deployed in force to calm crowds.

Election authorities said the problems were largely limited to Port-au-Prince. By early afternoon, all polls across this country of 8.3 million were open, said U.N. spokesman David Wimhurst.

Most voters waited in lines for hours at polling stations in the capital of the Western Hemisphere's poorest nation, but some angrily roamed the streets, fuming at being turned away because of a myriad of problems.

"If these elections are not fair and if the person whom the population wants doesn't win, houses will burn and heads will be cut off," said Jean Pierre, an unemployed 33-year-old man who would not give his last name.

The words recalled the battle cry of army Gen. Jean-Jacques Dessalines, who led a bloody rebellion against French troops and colonists in 1802: "Cut off their heads and burn their houses."

The stakes are huge — more than simply who will lead the country and who will occupy seats in parliament. Haiti, which has seen only one president complete his term in office, could implode if the elections go wrong, experts say.

In the aftermath of a February 2004 rebellion that toppled President Jean-Bertrand Aristide, gangs have gone on a kidnapping spree and the country's few factories are closing because of security problems and a lack of foreign investment.

U.S. officials have warned that a collapse could trigger another wave of Haitians migrating aboard boats to the United States. That occurred in 1994, prompting Washington to send troops to Haiti to restore Aristide to power, three years after he fell to a military coup.

In the northern town of Gros Morne, a Haitian policeman shot and killed a man in line at a polling station, Wimhurst said. A mob then killed the police officer, he said. There were two other reported deaths in polling stations in the capital — two elderly men who collapsed while waiting in line.

Preval — who has the backing of many supporters of Aristide — said if he wins, Haitians must recognize their country is in dire straits and should not set their expectations too high.

"We will not be able to do everything right away," he said. "But we are determined to do our best and raise the standard of living for the people of Haiti."

Acting U.S. Ambassador Timothy Carney applauded the vote.

"Today was a triumph for the Haitian people," he said. "It was a ragged start. The Haitian people turned out in force, clearly believing security was in place. They made it work. They waited in line patiently."

Wimhurst blamed the problems on poor planning and a lack of trained workers.

Underscoring the difficulty of holding elections in a country with a ruined infrastructure — including roads — mules transported some election materials to areas where U.N. helicopters were unable to land. The vote has been postponed four times since October.

Besides Preval, other top presidential contenders are Charles Henri Baker, 50, whose family runs factories that assembles clothing for export, and Leslie Manigat, 75, who was president for five months in 1988 until the army ousted him.

Also running are a former rebel in the insurgency that forced Aristide from office in February 2004 and a former army officer accused in the death of a Haitian journalist. If no candidate wins a majority, a March 19 runoff would be held between the top two candidates.