"
He signaled the irregularities could delay the publication of final results while an investigation got underway.
"We are sure of having won in the first round," Mr Preval said at a
news conference near the Haitian capital, Port au Prince.
"[But] we are convinced there was massive fraud and gross errors that affected the process," he said.
They were his first significant comments since last Tuesday's vote.
Results published Tuesday appear to support Mr Preval’s claim, showing he had 48.7 per cent of the vote with 90 per cent counted.
He needs a simple majority to avoid a run-off.
"If they publish these results as they are, we will contest them and if Lespwa (Preval's political movement) contests them, the Haitian people will contest them," Mr Preval said.
Mr Preval had a message for his supporters who had paralysed Port au Prince with flaming blockades and street demonstrations.
He has called on them to take down the barricades to allow people to get to work and school.
"Continue to protest, but respect the rights of others," he said.
Port-au-Prince is regarded as chaotic at the best of times but it seemed more peaceful Wednesday.
Some roads remain blocked by rocks, tree branches and other debris but traffic was moving.
The largely peaceful protests were marred by eyewitness accounts claiming Jordanian UN peacekeepers killed two people when they opened fire in Tabarre, just north of the capital.
Haiti's interim government has pleaded with Haitians to stay calm as election officials counted the last 10 per cent of ballots.
The government was appointed after Jean Bertrand Aristide fled the impoverished Caribbean nation in the face of an armed rebellion and under intense international pressure to quit.
"The people elected Preval. I respect their will," said Presidential candidate Dany Toussaint on local radio.
"I recognise they did not vote for me."
Mr Toussaint only won about 7,000 of more than two million votes cast.
Other presidential candidates also conceded Mr Preval had won.
They include Chavannes Jeune, who is running fourth, former
Port-au-Prince Mayor Evans Paul, who won just over two per cent of the vote, and ex-Prime Minister Marc Bazin, who took under one per cent.
Another ex-president, Leslie Manigat, was in second place with
11.8 per cent and industrialist Charles Baker, seen as the candidate of the wealthy elite, was third with 7.9 per cent.
Mr Preval said he would not need to win the presidency in a second round of voting scheduled for March 19.
He cited an independent tabulation by the National Democratic Institute, a US-based nonprofit organisation, which showed he had carried 54 per cent of the vote.
"The Haitian people know we have won in the first round," he said, adding that the government had agreed to hold off publishing results until his claims could be investigated.