Although polls opened at 6 a.m. Eastern time, some stations were still closed two hours later in the capital, Port-au-Prince, said U.N. spokesman David Wimhurst, Reuters reported.
U.N. peacekeepers in armored personnel carriers were on patrol to guard against violence in the Caribbean nation of about 8 million.
About 1,000 people lined up before dawn at one polling station in the Port-au-Prince area of Delmas, waiting for election officials to open the doors.
Hundreds of others formed a line along trash-strewn streets at another polling station in the poor downtown neighborhood of Bel-Air, according to Reuters.
"Haitians are mobilized for change, that's why there's so many people in the street this morning," said Jean Joseph, 44, on his way to vote.
The front-runner is former president Rene Preval, a 63-year-old agronomist and protege of Aristide. Preval led the country from 1996 to 2001.
The other top contenders among the 33 candidates are businessman Charles Henri Baker and Leslie Manigat, who was president for five months in 1988 until a coup ousted him.
Guy Philippe, one of the leaders of the rebel group that had a hand in driving Aristide from power, also is a contender.
If no candidate wins a majority, the top two finishers will compete in a runoff on March 19.
Hundreds of candidates also are running for 129 parliamentary seats.
Official results may not be known for three days, according to the Associated Press.
The elections are seen as a move to restore democracy, but the island nation is still plagued with judicial, security, health and corruption problems that many observers say need more than a quick electoral fix.