UN peacekeepers in Haiti have intervened to restore order in the capital, Port-au-Prince, after three policemen were killed in protests.
UN troops are guarding food distribution centres in Haiti
Demonstrators called for the return of ex-President Jean-Bertrand Aristide, who fled the country earlier this year.
Looting and armed gangs are threatening the relief effort in the city of Gonaives, ravaged by Hurricane Jeanne.
The crisis is overstretching the UN mission to stabilise the island, officials say.
Gunfire broke out as Mr Aristide's loyalists emerged from the slums promising that he would return from exile in South Africa.
They were marking the 13th anniversary of his being ousted from power in 1991 by Haiti's army.
Interim Prime Minister Gerard Latortue vowed to capture those responsible for the violence, in which some civilians were also reportedly hurt.
Hurricane Jeanne, which hit Haiti last month, killed more than 1,500 people and left 200,000 starving and homeless.
The UN is launching an appeal for $30m in emergency aid for the impoverished country.
UN peacekeeping forces say they turned away about 150 armed men who tried to enter Gonaives on Wednesday.
Witnesses say the men, said to be former Haitian soldiers, told UN troops they wanted to take food to those residents affected by the storm.
The men accused the UN mission of not doing enough to protect local people, and some said they would try to enter the city again.
"No foreigner has the right to tell us to put down our arms," one rebel leader told the Associated Press news agency on Thursday, after returning to the capital.
"We went there because wherever security is needed, we'll be there."
The UN says it needs a 1,000 more peacekeepers to help to protect food convoys heading for Gonaives.
More than 1,500 are known to have died in the port city while about 900 are missing, many of them possibly washed out to sea during the storm nearly two weeks ago.
People in Gonaives are desperate for food and clean water
Armed gangs roam the flooded streets, shooting people in their search for food and looting aid lorries.
Tonnes of food aid are piled up in a warehouse guarded by peacekeepers.
"The problem is not a lack of food. It's security and a
lack of trucks," Ricardo Mena of the UN's Office for the Co-ordination of Humanitarian Affairs (OCHA) said on Thursday.
An AP correspondent who visited a slum in the city found mud 1.5m (five feet) deep still mired streets where unclaimed dead bodies have been buried in mass graves.
A lack of clean drinking water is adding to the food crisis in Gonaives.
Volunteer doctors have spoken of appalling conditions as they try to help the storm survivors in makeshift clinics.
Amputations have been carried out under primitive conditions, many people have infected wounds, and there are widespread cases of diarrhoea.