Balloting began in most of the country shortly before 6:00 am (1100 GMT), as scheduled. However, some polling stations opened late.
Authorities have banned motorbike traffic and alcohol sales on election day as extra security measures.
Front-runners among the 18 candidates include Jude Celestin, an engineer supported by Preval; academic and former first lady Mirlande Manigat; and Michel Martelly, a popular singer widely known as "Sweet Micky".
The election comes as Haiti battles a cholera outbreak that has claimed at least 1,648 lives. It is also the first election since a devastating 7.0-magnitude earthquake in January killed more than 250,000 people.
Haiti correspondent Isabeau Doucett said the elections had been met with a lot of resistance since they were first postponed in January and turnout was expected to be very low.
“There are so many reasons why so many Haitians are not going to vote, or don’t want to vote or might go and vote with a spoiled ballot,” said Doucett.
“The people are still living in very, very bad conditions. There’s cholera and people are dying in the street. There are trucks going round picking up bodies and dumping them in the capital and the capital has not been the place that’s been hardest hit.”
She says many Haitians were protesting in the street after candidates were excluded from the vote on technical grounds such as the most important political party Fanmi Lavalas.
“Whoever wins – not that I think there will be a very clear winner - will be a very, very weak government and it will be overseeing all sorts of reconstructions for the next five years, all sorts of huge projects, all sorts of huge dilemmas,” says Doucett.