Violence broke out in some of Port-au-Prince's poorest neighborhoods, which were carried by Maryse Narcisse's Fanmi Lavalas party.
Businessman Jovenel Moise's election as Haiti's next president was challenged by losing rival candidates, signaling more political upheaval in the troubled nation as sporadic protests erupted in the capital.
Violence broke out in some of Port-au-Prince's poorest neighborhoods, which were carried by Maryse Narcisse's Fanmi Lavalas party, against Moise, the man former president Michel Martelly chose to represent his party.
Police launched tear gas grenades to disperse a crowd of hundreds of her supporters who called the vote an "electoral coup" and reacted to the advancing officers by throwing stones.
"We didn't vote in secret. All of the working-class neighborhoods in the country... voted for Maryse, but the results they announced were an electoral coup," said Rose-Marie Rosilus.
Rosilus lives in Bel Air, a neighborhood that has historically been a bastion of former leader Jean-Bertrand Aristide, who heads the Lavalas party.
"We will stay out in the streets until the electoral council gives us our true results," added Rosilus, who brought lemons to alleviate the effect of the tear gas.
There were no reports of violence elsewhere in the capital or across the country.
Moise, a 48-year-old political novice and entrepreneur who worked in agriculture mostly growing bananas, earning him the nickname "banana man," wants to lift the Caribbean country out of poverty by reviving its agricultural sector.
"We will need to mobilize all the resources of the country, as I repeated during my campaign: men, land, sun and rivers to put food on people's plates and money in their pockets," Moise said late Monday, shortly after the preliminary results of the first-round election were announced.
But his rivals' challenges over the vote's legitimacy could run those plans into the ground.
The preliminary results showed that Moise won the election outright, garnering 55.7 percent of the vote, thus barring the need for a second round. But he lacks much popularity, with only 21 percent of eligible voters casting their ballots.
Jude Celestin, who ran as a candidate of the opposition LAPEH and came in second with 19.52 percent, is refusing to accept the outcome, along with fellow candidates Moise Jean-Charles (11.04 percent) and Narcisse (8.99 percent).
"We are saying there was cheating and we will see who cheated," Celestin told AFP, without naming Moise directly.
Indicating he would pursue the matter in the courts, Celestin claimed the results "do not reflect the popular vote."
But election observers from the Organization of American States said the results were "in line" with data they collected at polling stations.
The OAS electoral mission "calls upon all actors to respect the will of the Haitian citizens, expressed emphatically through the polls," it said in a statement.
Interim leader Jocelerme Privert urged for all parties to remain calm in order to "safeguard peace and political stability."
In a statement from his office, Privert said the elections were "crucial for Haiti's political, economic and social future. It's an important step toward political and institutional stabilization in the country."
Provisional Electoral Council president Leopold Berlanger cautioned that the results were preliminary and final results would not be confirmed until December 29.
Haitian law offers candidates the opportunity to challenge the results in electoral courts.
The drawn-out proceedings are slowing down the return to constitutional order in a desperately poor country already beset by political upheaval.
Three of the council's nine members refused to sign the result, signaling a potential conflict over the outcome.
The long-delayed November 20 election took place without major incident. Haiti's vote was originally held in October 2015, but the results were scrapped after an independent commission found massive fraud.
Moise was initially said to have won the October 2015 election's first round with approximately 33 percent to 25 percent for Celestin, who denounced the results as a "ridiculous farce" before they were scrapped.
Fatigued by the long-delayed vote, the majority of Haitians report that they do not believe an election can lift Haiti -- where more than 60 percent of the population survives on less than $2 per day -- out of extreme poverty.
Nearly 6.2 million people were eligible to vote in the Americas' poorest country, parts of which are still struggling to recover from a devastating hurricane.
More than 800,000 people in Haiti currently need emergency food assistance, according to the United Nations.