Haiti's international airport was shut and the offices of its ruling coalition government were set alight amid violent clashes after electoral authorities announced that President René Préval's hand-picked successor would stand in a runoff election.
An announcement that Jude Celestin, a government-backed technocrat, had won enough votes in the 28 November vote to stand in next month's second round led to thousands of protesters rampaging through the streets of Port-au-Prince, and clashes with UN peacekeepers.
Celestin, who came second with 22.48% of the vote, will face a former first lady, Mirlande Manigat, who won 31.37%. The runoff was mandated because no one won more than 50% in the first ballot.
The US embassy in Port-au-Prince issued a swift statement questioning whether the results were consistent with "the will of the Haitian people".
Celestin pipped a singer and musician, Michel "Sweet Mickey" Martelly, who was widely expected to make it into the second round. Martelly supporters said their man was robbed.
Flaming barricades were set up in the neighbourhood where the tallies were announced after a day of delay and tension. "If they don't give us Martelly and Manigat [in the second round], Haiti will be on fire," one protester, Erick Jean, told Associated Press. "We're still living under tents and Celestin wastes money on election posters."
"We want Martelly. The whole world wants Martelly," said James Becimus, a 32-year-old protester near the US embassy. "Today we set fires, tomorrow we bring weapons."
Martelly urged supporters to continue their protests, but peacefully. In a radio address, he also told supporters to watch out for "infiltrators" who might try to incite violence.
"Demonstrating without violence is the right of the people," he said.
Haiti's Radio Metropole reported that at least one activist was killed in Les Cayes, about 120 miles west of Port-au-Prince in the country's southern peninsula.
Evidence of widespread fraud, intimidation and confusion put a question mark over last month's poll and the credibility of the provisional electoral council, which many consider an instrument of Préval.
Celestin's well-funded campaign covered the quake-battered country in bright yellow posters but he appeared to be damaged by the unpopularity of Préval, who is not allowed to stand again.
The National Observation Council, an election watchdog financed by the European Union, was among several groups that said Celestin would be eliminated if votes were honestly counted.
Martelly, an outsider once best known for his bawdy lyrics, was given 21.84%, less than 1% (6,800 votes) behind Celestin.
The singer made no immediate response and cancelled a press conference for security reasons but he had warned of uproar if the president's candidate made it into the second round. Some foreign diplomats floated the idea of allowing three candidates in the second round if results in the first were tight.
The UN mission in Haiti and a joint Organisation of American States/Caribbean Community election observers had given an initial endorsement of the vote, despite acknowledging irregularities.
But the UN secretary-general, Ban Ki-moon, said on Friday that the electoral irregularities were "more serious than initially thought" and called for a "Haitian solution" to avert violence.
