By LYDIA POLGREEN
 
PORT-AU-PRINCE, Haiti, March 3 — United States marines took to the lawless streets of the capital on Wednesday, patrolling in light armored vehicles and Humvees late in the afternoon in their first direct attempt to quell the violence that has racked the city since the former president, Jean-Bertrand Aristide, fled Sunday.

The rebel leader, Guy Philippe, announced that he would disarm his men and leave the city, effectively ending a power struggle over who would control the streets. He agreed to disarm after a 10-minute meeting with American military commanders in the capital.

Haiti's interim president, Boniface Alexandre, spoke to the nation for the first time since Sunday, urging Haitians to unite to defuse the nation's crisis.

"I call on all sides," Mr. Alexandre said in a radio address in Creole, "on all citizens of goodwill, to join us in a reconciliation process and to help each other in rebuilding our nation."

But the political crisis deepened. Prime Minister Yvon Neptune declared a state of emergency, which effectively suspended many of the rights guaranteed by Haiti's Constitution. He said the government was struggling to perform its most basic functions.

"I have remained in my post to ensure an orderly transition," Mr. Neptune said at a news conference on Wednesday afternoon. "However, this is not the case for the ministers of my government. Those who did not leave the country had to go underground for fear of violence to them and their families."

Mr. Alexandre said he had fired the police chief and named an interim chief, and had frozen all state accounts except the one controlled by Mr. Neptune.

It was a day of violence and fear in the capital. Rebels and the police battled Aristide loyalists with volleys of gunfire in the squalid slums that sent thousands of people fleeing.

Jeudi Backner, an electrician, was working at a shop on Jean-Jacques Dessalines Boulevard when he heard shots ring out down the street. A dozen rebel soldiers and policemen opened fire on a pack of armed pro-Aristide militants, known as chimères, Mr. Backner said.

"They were just shooting and shooting," Mr. Backner said as he cowered in a doorway a few blocks from the shootout. "The police went to disarm the chimères, but they didn't want to drop their guns."

It was a scene played out a half-dozen times on Wednesday, until 60 to 70 marines took to the streets in about a dozen vehicles. It was a show of force seemingly intended to quell the confusion and anarchy that have reigned since Mr. Aristide, the country's first democratically elected president, left office early Sunday under pressure from the United States and France, and from the armed rebels led by Mr. Philippe.

Mr. Aristide flew to the Central African Republic; he is expected to leave there soon for permanent asylum elsewhere.

Mr. Philippe — appearing more subdued than he did on Tuesday, when he marched through the capital declaring the country was "in his hands" — said he would soon leave the capital and withdraw his men.

"Now that there are foreign troops promising to protect the Haitian people," he said at a news conference, "we will lay down our arms."

The marine contingent more than doubled on Wednesday, to 1,100. There are also 600 French soldiers here, military officials said, and 200 more are to arrive Thursday.

The marines were received with both wariness and gratitude. Small crowds gathered wherever they patrolled, and dozens of people clung to the green metal fence outside the National Palace to ogle three armored vehicles parked on the lawn.

"The last time they came they didn't do much," said Raymond Beauséjour, a 34-year-old plumber who has been unemployed for five years. "This is not the kind of aid we need. They should help us build schools and clinics and to get jobs."

But Yolette Estimable, who lost thousands of dollars in a government-backed pyramid scheme, said she was glad the marines were there. "They will bring us security," she said. "They will control the chimères and let us live in peace."

Security was on the minds of people living in the largest and most squalid slum, Cité Soleil, a stronghold of support for Mr. Aristide, where residents said they feared an incursion by rebel troops. The leaders of the rebel army include members of Fraph, a militant group that massacred thousands of people, many of them Aristide loyalists, in Cité Soleil and other slums during the rule of the military junta that deposed Mr. Aristide in 1991.

"The gift we received from Fraph was pigs eating the bodies of our brothers and sisters," said Paul Virel, a 32-year-old militant who vowed to fight rebels who tried to suppress Cité Soleil's armed fervor for the deposed president. "Mothers would wake up in the morning to find their sons dead. Fraph will never come here again. We would rather die stopping them than let Fraph come back."

But Elamise Misidor, who lives in a tin shack at the end of a rutted road flooded with putrid green water, said it made little difference who sat in the National Palace.

"It does not matter to me," she said, spooning rice from a metal plate into her daughter's mouth as she squatted on a rock. "There is no life here. We have no money. We have no food."

A gang leader going by the name Billy said he had thousands of men armed with M-16 rifles, handguns and other weapons, ready to defend Cité Soleil.

"We are not going to wait for them to come and kill us," he said, an automatic handgun tucked into the hip pocket of his black Levis, a broad grin under the brim of his straw hat. "We'll take care of Guy Philippe. You will see."