Haitian authorities have retaken a key northern city from rebels opposed to President Jean-Bertrand Aristide.
Television pictures showed people looting from shipping containers
Police, backed by helicopters, entered the port of Saint-Marc, about 65 miles (105km) north of the capital Port-au-Prince, city residents said.
Prime Minister Yvon Neptune flew to the city and urged all sides to help restore calm.
Earlier, Mr Neptune accused the civil opposition of trying to mount a coup as unrest continued to spread in Haiti.
About 10 towns in northwestern Haiti have been affected by the five-day street violence in which some 40 people have been reportedly killed.
An opposition spokesman denied backing the unrest and called for foreign intervention to avert civil war.
France said it was very worried about the developments and appealed to both sides to end the violence immediately.
"Our officials on the ground are working together with the other diplomatic and consular missions that are present," French Foreign Ministry spokesman Herve Ladsous said.
The United States expressed great concerns about the continuing violence, urging Haitians to respect the law.
"The problems of Haiti will not be solved by violence and retribution," State Department spokesman Richard Boucher said.
United Nations Secretary General Kofi Annan said the situation was being monitored closely.
"We will be stepping up our own involvement fairly soon," Mr Annan said.
"The national police force alone cannot re-establish order," Prime Minister Neptune told the Associated Press agency, as he was inspecting the charred remains of the Saint-Marc police station torched by the rebels.
Earlier, Mr Neptune said the opposition should play a role in stopping the violence and help the country to hold elections.
Television pictures earlier showed crowds - said to number hundreds - looting food and goods in the city.
Opposition spokesman Andy Apaid told the BBC that President Jean-Bertrand Aristide must stand down, adding that the international community's help was needed to "make sure the message gets through".
But, he added, the opposition did not support the violence sweeping through Haiti.
Further north, police have withdrawn from the rebel-held city of Gonaives.
Most of the city's 200,000 inhabitants are also believed to have fled.
Police had been trying to regain control of Gonaives since rebels - who describe themselves as the Gonaives Resistance Front - seized control of it on Friday, and nine people reported killed in clashes.
Opposition groups are calling for the president's resignation, saying he stole the 2000 election which returned him to power.
They also accuse him of corruption and human rights violations.
President Aristide has offered to hold parliamentary elections but insists he will serve out his second term in office, which ends in 2006.