Haitian protesters have set fire to buildings, clashed violently with UN peacekeepers and shut down the capital's airport to express their anger at the choice of candidates for a second round of presidential elections.

The violence that spread through the country began Tuesday night, after it was announced that government-backed candidate Jude Celestin had advanced to the second-round run-off.

Protesters set fire to the headquarters of the Unity party, which was recently founded by the increasingly unpopular outgoing President Rene Preval, who has endorsed Celestin as his successor.

Witnesses reported the rare sight in Haiti of firefighters arriving on the scene, but late in the day piles of charred rubble continued to smoulder.

Protesters across the country waved pink signs emblazoned with the image of popular candidate Michel "Sweet Micky" Martelly, a singer, who came within one per cent of Celestin's vote total, but was apparently not included in run-off plans.

"We want Martelly. The whole world wants Martelly," said protester James Becimus near the U.S. Embassy. "Today we set fires, tomorrow we bring weapons."

Martelly supporters also lit smaller fires with government campaign posters and engaged in angry confrontations with UN soldiers.

Outside the provisional electoral headquarters in the suburb of Petionville, protesters hurled rocks at the UN troops, who responded by firing tear gas into the crowd. Clouds of smoke wafted over a nearby earthquake-refugee camp, which sent women and children fleeing from their tents.

Martelly took to the airwaves Wednesday to urge his supporters to keep their demonstrations peaceful. He also used his radio address to warn of "infiltrators" who may try to spark violence.

"Demonstrating without violence is the right of the people," he said. "I will be with you until the bald-head victory."

Preval, whom Haitians have widely criticized for the painfully slow reconstruction effort after last January's earthquake, urged candidates to ask their supporters to stand down.

"This is not how the country is supposed to work," Preval said in a live radio address. "People are suffering because of all this damage."

Much of the anger on the streets was linked to the decision to put Celestin through to the second round. Celestin's purported success is at odds with the prior reports by a local election monitoring group, which predicted he would likely be eliminated. Officials at the U.S. Embassy in the capital, Port-au-Prince, have also questioned the results.

The demonstrations also poured fuel on the firestorm of allegations surrounding the Nov. 28 election, which saw confirmed incidents of ballot-stuffing, violence and intimidation.

Countless Haitians were also prevented from casting their ballots, as voter lists contained many names of residents who died in the earthquake. Just over one million people cast their votes out of about 4.7 million registered voters.

Canadian Foreign Affairs Minister Lawrence Cannon says Canada is "concerned with the events" and urged "political actors to fulfill their responsibilities in the interest of the population, and demonstrate a firm commitment to peace, democratic principles and the integrity of the electoral process."

Haitian election officials announced late Tuesday that former first lady Mirlande Manigat and the government-backed Jude Celestin will advance to a second-round runoff.

The preliminary election results gave Celestin a 6,800-vote edge over Martelly for second place. Manigat was leading with 31.4 per cent of the vote. Celestin had 22.5 per cent, while Martelly had 21.8 per cent. The run-off is scheduled for Jan. 16.

The announcement brought chaos to streets of Port-au-Prince overnight Tuesday, as well as to the north and west in the city of Cap-Haitien.

Local radio also reported protests in the southern town of Les Cayes in which protesters, many of them Martelly supporters, burned down government buildings. Late Wednesday, Haiti's Radio Metropole reported that at least one demonstrator had been killed in Les Cayes.

Craig Kielburger, the well-known co-founder of Canada's Free the Children charity, was in Port-au-Prince when election officials announced their runoff plans on Tuesday night. After the news was announced, Kielburger said protesters quickly demanded a recount.

"Immediately, people took to the streets and they started chanting right in front of us," Kielburger told CTV's Canada AM by telephone Wednesday morning.

Meanwhile, American Airlines -- the main international carrier in Haiti -- has halted all flights in and out of the capital amid the protests. An airline spokeswoman told The Associated Pres airport employees were unable to get to work Wednesday because of the demonstrations.

With files from The Associated Press