Gunmen opened fire on a crowd of 10,000 people that had gathered in the capital, Port-au-Prince, to celebrate Mr Aristide’s downfall.
Witnesses blamed militiamen supporting Mr Aristide for the shooting.
US and French troops had been at the march, in an effort to prevent clashes between the two groups.
Meanwhile, Mr Aristide, on Monday insisted that he remains the elected president of his country.
Speaking at a news conference in the Central African Republic he called for “peaceful resistance” in Haiti and the restoration of democracy.
The BBC’s Daniel Lak in Port-au-Prince says there had been a festive atmosphere before the trouble broke out.
The peacekeepers were nowhere near where the shooting was
Witnesses said pro-Aristide gunmen known as Chimeres had come out of the slums around the square and opened fire from buildings or the top of a hill.
A Spanish television journalist, Ricardo Ortega, was among those killed.
At least 20 people were also said to be injured – among them other foreign reporters.
Hospitals sent out appeals for blood donations and medical staff to cope with the casualties.
Witnesses criticised US-led troops for failing to prevent the violence.
The Americans are only there to protect those who helped oust Aristide
One man speaking over a truck loudspeaker told US marines: “People are dying every day in this country. You have to do something about it.”
The 2,500 French and US peacekeepers – who have arrived in Haiti in recent days – had restored order in central Port-au-Prince but have avoided outlying slums where Mr Aristide has many supporters.
Maj Richard Crusan of the US marines told the BBC World Service that they did respond during the unrest to secure a building thought to have contained gunmen.
“It was pretty chaotic,” he told the World Today programme, saying that some semblance of order was soon restored.
US marines said they wanted to support the Haitian police
Earlier, the pro-Aristide camp postponed a planned demonstration on Sunday, saying it had been offered no protection.
One supporter told AP: “The Americans are only there to protect those who helped oust Aristide.”
The former president’s supporters said their protest would now be held on Monday.
Cheers and demands
Earlier in the day, the anti-Aristide crowds tore down a billboard featuring the ousted president’s face, carried it to the presidential palace and set it on fire.
Rebel leader Guy Philippe was feted by supporters
Some demonstrators demanded that the former president should be put on trial.
“His people must also be tried for looting state coffers and murdering civilians,” opposition leader Evans Paul said.
Opposition leaders have been pressing for the replacement of Prime Minister Yvon Neptune, who has strong ties to the deposed president.