PORT-AU-PRINCE, Haiti - U.S. Marines are investigating the attack on thousands celebrating the ouster of President Jean-Bertrand Aristide that killed at least five people and led Marines to return fire in the first armed action of their mission to Haiti.

A few doctors without enough medication or staff struggled today to treat dozens of injured from Sunday's protest, despite the dramatic arrival of a French Air Force helicopter that landed on a major road to deliver emergency supplies to Port-au-Prince's main private hospital.

Sunday's was the worst attack since Aristide fled Feb. 29 and involved the first gunfire by U.S. forces sent a week ago to stabilize Haiti.

Among the five people killed were Spanish television correspondent Ricardo Ortega. Dozens were injured, including South Florida photographer Michael Laughlin, 37.

What protesters called a "victory march" began with a couple hundred people in Port-au-Prince's Petionville suburb, with Haitian police in the lead along with a convoy of U.S. Marines in five Humvees mounted with machine guns. Two truckloads of French legionnaires were in the tail.

It was a test of Haiti's shaky democracy in the aftershock of Aristide's flight - prompted in part by a monthlong popular rebellion - and of newly arrived U.S. and French peacekeepers.

Aristide militants who have attacked protesting opponents in the past said they too would march Sunday to demand the return of the exiled leader who says he was forced from power by the United States.

A confrontation seemed inevitable.

"Try Aristide! Jail Aristide!" protesters yelled, demanding he stand trial for alleged corruption and killings committed by his militant supporters.

As the number of protesters swelled to thousands, the peacekeepers got hemmed in.

When marchers converged on the central Champs de Mars plaza, gunfire erupted. Many witnesses said they saw Aristide militants start the shooting.

U.S. Maj. Richard Crusan said three Marines fired in the direction of the attack.

"We are unaware that any action was taken to other reports of shooting. We are still reviewing that information," he said.

Many of Sunday's victims were shot with high-velocity bullets from weapons like M-16s and M-14s, orthopedic surgeon Ronald Georges said.

Prime Minister Yvon Neptune condemned the killings. He also defended the Marines' return of fire, saying they abided by "rules of engagement (that) permit that they use proportional force."

Neptune - an Aristide appointee whom protesters also demanded should go on trial - ordered police to search for perpetrators and "start disarming all who carry illegal weapons."