At least one person has been killed and several injured in clashes in Haiti, as tensions mount over the results of last Tuesday's election.
Supporters of presidential front-runner Rene Preval manned burning roadblocks in the capital and occupied a hotel, demanding he be declared the winner.
Witnesses said UN peacekeepers had opened fire on protesters, but the UN said its troops had fired in the air.
Mr Preval returned to Port-au-Prince as the prospect of a run-off vote grew.
The former president's share of the vote fell further on Monday, according to electoral officials.
With nearly 90% of the ballots counted, Mr Preval has 48.7% of the vote - just short of the 50% required to win outright.
Another ex-leader, Leslie Manigat, has 11.8%, while industrialist Charles Henri Baker has 7.9%, officials say.
Mr Preval used to be an ally of ex-President Jean-Bertrand Aristide, who was forced out of power in 2004.
He has inherited Mr Aristide's strong support among the poor.
Thousands marched in Port-au-Prince for a second day on Monday, as it became increasingly apparent that Mr Preval might face a run-off on 19 March.
Angry protesters say electoral officials have tampered with the vote count to keep him from a first-round victory.
Final results had initially been scheduled for Sunday evening - but as it stands now they may not be known for a few days.
Burning tyres and roadblocks paralysed the capital's streets, as protesters let only journalists and Red Cross vehicles pass, the Associated Press news agency reports.
Witnesses told local media that UN peacekeepers had fired into crowds of demonstrators massing in the middle class Tabarre district, killing at least one.
But UN spokesman David Wimhurst insisted troops had only fired two shots in the air. "No individuals were wounded by UN peacekeepers," he said.
The foreign minister of Brazil, which heads the UN troops contingent in Haiti, has meanwhile called for the UN Security Council to discuss the post-election unrest.
Celso Amorin is said to have made the request during a phone conversation with his US counterpart, Secretary of State Condoleezza Rice.
'Free and fair'
In the Petionville neighbourhood, thousands of screaming Preval supporters poured into the Montana Hotel, where election officials have been tallying results.
UN peacekeepers barred access to the election centre and - after hours spent chanting pro-Preval slogans and frolicking in the hotel pool - the crowd finally dispersed peacefully, AP reports.
Mr Preval, who is leading more than 30 presidential candidates, has returned to the capital from his home town of Marmelade.
Reports suggest the former leader, who has been in talks with UN and international officials, may appeal for calm.
He held a 61% lead when the first results were released late last week, but his share of the vote has since dropped.
There have been accusations of ballot mishandling - but the head of Haiti's electoral council has denied the allegations.
International observers have deemed the election free and fair, despite saying there were some minor procedural irregularities.
Haiti - the poorest country in the continent - is choosing a 129-member parliament as well as a new president.