Some polling stations opened late others not at all in the first presidential election since a bloody revolt two years ago pushed this bloodied, impoverished nation toward total collapse.
Although polls were scheduled to open at 6am local time (11am Irish time), some did not open until hours later. Because of organisational problems, voting hours were extended.
Rosemond Pradel, the secretary-general of Haiti's nine-member electoral council, did not give a new closing time but said there were no plans to extend voting hours to Wednesday.
A shortage of workers, missing ballots and other problems delayed the opening of some voting stations, including those used by people from Cite Soleil, a volatile shantytown at the northern edge of the capital, Port-au-Prince.
"There's some frustration and anger on the voting lines," said David Wimhurst, spokesman for the UN, which has 9,000 peacekeepers trying to maintain order.
"People have been waiting several hours and in some cases they haven't got inside."
More than 5,000 people waited to vote at a polling station near Cite Soleil.
The polling station opened three hours behind schedule.
Earlier, gunshots could be heard from within the slum, which is home to some 200,000 people.
The election front-runner is former president Rene Preval, a 63-year-old agronomist who led the country from 1996-2001. He was expected to win support from many poor Haitians, including residents of Cite Soleil and backers of ousted President Jean-Bertrand Aristide.
The other top contenders among the 33 candidates are businessman Charles Henri Baker, 50, whose family runs factories in the assembly-for-export industry, and Leslie Manigat, 75, president for five months in 1988 until the army ousted him.
The field also includes 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, the top two finishers would compete in a March 19 run-off. Hundreds of candidates also are running for 129 parliamentary seats.
UN Secretary-General Kofi Annan said the 9,300 peacekeeping force will do all it can to ensure the vote is held freely and safely.