Mr Preval scored more than 90 percent of the votes in a large centre where residents of the notorious Cite Soleil slum cast their ballots, and unexpectedly held a strong majority in several voting offices in wealthier neighbourhoods.
At a school in the middle-class suburb of Petion-Ville, Mr Preval took 70 percent of the votes, according to results posted on the walls.
Electoral officials insist the results are not official as they still have to be verified and officially tabulated.
Senatorial candidate Marylande Manigat, the wife of one of Mr Preval's main rivals, former president Leslie Manigat, said Mr Preval had a very strong lead in Port-au-Prince and the capital's outlying areas.
Mr Preval, 63, once had close ties to Jean Bertrand Aristide, Haiti's last elected president, who resigned and fled the country with US and French help on February 29, 2004 as insurgents closed in on the capital.
A trained agronomist, Mr Preval was president from 1996 to 2001 and had served as prime minister under Mr Aristide for seven months in 1991.
He is often seen as a champion of the poor who make up 77 percent of the 8.5 million population.
He draws little sympathy from business leaders, many of whom said they favored Mr Manigat or industrialist Charles Henry Baker, who both trailed far behind Preval in voting centers of the capital surveyed by AFP.
On Wednesday, members of the 9,500-strong UN military and police force in Haiti escorted ballots to the heavily protected electronic tabulation center where electoral officials immediately got to work.
UN special envoy to Haiti Juan Gabriel Valdez and Jose Miguel Insulza, the secretary general of the Organisation of American States, as well as electoral observers, kept an eye on the proceedings at the tabulation office.
Many of the results from around the country were still en route to the capital on Wednesday, and some were carried by mule from mountainous, hard-to-reach areas.
Further reflecting the difficulties in staging elections in a country as poor as Haiti, much of the vote counting during the night was done by candlelight.
In some voting centres, the tallying continued Wednesday morning.
Heavily guarded by members of the UN force, known as Minustah, the election was free of a renewed explosion of political violence many had feared in this volatile country.
But the voting was marked by stampedes that reportedly left four people dead and several more wounded.
Throngs of people walked for hours in the absence of public transportation, and in several cases stormed the gates of offices that opened hours late.
Despite problems during the elections, which had been postponed four times since November, international observers hailed the fact that voting could be held in a country plagued by armed gangs and rampant poverty, and with a history of fraudulent elections and military coups.
"I am pleased that, compared to previous elections, yesterday was remarkably free from violence and I applaud the Haitian people for their commitment to restore democracy," said UN Secretary General Kofi Annan.
He appealed to Haitians to respect the outcome of the vote, urging them to "come together in a spirit of national reconciliation and dialogue in order to build strong democratic institutions and an inclusive governance system."
More than 3.5 million Haitians were registered to participate in the election that also renewed the 129-seat parliament.