A policeman and a civilian died of gunshot wounds, and four were wounded, after a clash errupted when voters tried to storm a polling centre in the town of Gros-Mornes in northwestern Haiti, radio Vision 2000 reported.
In the capital Port-au-Prince, one man was asphyxiated and another died of a heart attack as crowds rushed voting offices that were still closed hours after the presidential and legislative elections started, a UN official said.
More than a dozen others were injured, many of them trampled, in the capital and in other cities.
Officials of the 9,500-strong UN military and police force in Haiti also said 22 people were wounded, four of them seriously, when the wall of a voting centre collapsed in St Louis du Nord.
Polling station delays
More than three million Haitians were eligible to vote to choose a successor to Jean Bertrand Aristide, the country’s last elected president who resigned and fled the country two years ago.
Many voters woke up long before dawn and walked for hours to get to voting centers, only to stand in huge lines, waiting for electoral officials or materials to arrive.
Authorities appealed for calm, apologised for delays and said that by noon, the problems were resolved in many of the voting stations.
Preparations for the presidential and legislative elections already had been marked by a series of setbacks that caused the voting, initially scheduled for November, to be postponed four times.
International observers hailed the very fact that the voting could be held a country terrorised by armed gangs, plagued by rampant poverty, and with a history of fraudulent elections and military coups.
A 9,500-strong UN military and police force, which has been in Haiti since 2004, stepped up security following an upsurge of violence in recent months.
Tension remained high as voting began on Tuesday, and thousands of people staged a protest march through the Bel Air slum, decrying delays and the fact that residents of the tough Cite Soleil slum were forced to cast ballots in neighbouring areas due to security concerns.
The protesters, brandishing their voter ID cards, also chanted the name of former president Rene Preval, the frontrunner in opinion polls.
In the dirt-poor slums of the capital that have been hotbeds of violence, many back Mr Preval, 63, a former ally of Mr Aristide.
Mr Aristide was widely popular among impoverished Haitians, who make up 77 percent of the 8.5 million population and who often blame the United States, France and Canada for his departure.
"We hope Preval will win, we want things to change at all levels -- jobs, education, stability," said Clairmont Ronald, 35 and unemployed, as he waited in a long line to cast his ballot.
But Mr Preval draws little support among better-off Haitians, who seem to favor industrialist Charles Henry Baker, 50 or former president Leslie Manigat, 75.
"We should not be afraid of change, we should not return to the old ideas," said businessman Alex Turner, 53, who said he would vote for Baker, but would wait till the crowd thins out.
Opinion polls ahead of the election gave Mr Preval a lead of at least 27 percent but the surveys' reliability was uncertain.
Should no candidate obtain 50 percent of the vote, the frontrunner would face off with the second place candidate in a run-off in March.
Among the long-shot candidates is Guy Philippe, 37, a former police commissioner who led the insurgency that played a key role in pushing out Mr Aristide two years ago.
More than 800 voting centres have been set up for the election that will also renew the 130-seat legislature.