Some Preval supporters on Sunday threatened the demonstrations could turn violent if Preval is not declared the first-round winner, accusing the electoral council of manipulating the count. Election officials deny any wrongdoing.
Robert Antoine, 23, from the Bel-Air slum, said: "If they take the election from Preval, it's not going to go smoothly. The people voted massively for Preval, and it seems the electoral commission is playing games with the results."
In the seaside slum of Cite Soleil, another Preval stronghold, about 1000 demonstrators wearing Preval T-shirts and blowing horns held a rally and prepared to march to the electoral council's offices.
In the Port-au-Prince area of Delmas, about 6000 protesters, many wearing Preval hats and waving campaign posters, boisterously marched down a main street, singing: "Our hearts beat for Preval!"
Other Preval supporters were planning to block roads in Jeremie, a town about 100 miles southwest of Port-au-Prince, local radio reported.
Preval was leading a field of 33 candidates with 49.1 per cent of the vote, five days after Haitians went the polls to elect a new president. Officials say 75% of the ballots from Tuesday's elections have been counted.
Barring a change, Preval would fall just short of the 50% plus one vote he needs to avoid a 19 March runoff with the runner-up. Leslie Manigat, also a former president, was second with 11.7 per cent of the vote.
"If they take the election from Preval, it's not going to go smoothly"
Officials said final results could be released as early as late on Sunday, although the count has been slowed by frequent delays. An estimated 2.2 million people cast ballots, or 63% of registered voters.
Thousands of Preval partisans flooded downtown Port-au-Prince and marched peacefully past the national palace on Saturday, declaring Preval the winner and demanding the results. Some shouted: "No Preval, no peace!"
The weekend protests were a break in the eerie calm that had settled over the normally tense capital since the vote. Nobel laureate Desmond Tutu presided over Sunday church services at Trinity Cathedral in Port-au-Prince and urged Haitians to be patient.
"They've started well, let them finish the race well," said Tutu, the retired Anglican archbishop of Cape Town, South Africa. "And I think they will, that they will be peaceful and that they will accept the results of the elections."
Desmond Tutu urged Haitians to
About 125,000 ballots - or 7.5 per cent of the votes cast - have been declared invalid because of irregularities, raising suspicion among Preval supporters that polling officials are trying to steal the election.
Haiti has been without an elected leadership and has been descending into anarchy since President Jean-Bertrand Aristide was ousted in a bloody rebellion two years ago. Trailing Preval and Manigat in the polls was businessman Charles Henri Baker.
Preval, 63, a former Aristide protege, declined to comment on the results while awaiting official word in Marmelade, his rural hometown.