By LYDIA POLGREEN and TIM WEINER
 
PORT-AU-PRINCE, Haiti, Feb. 29 — President Jean-Bertrand Aristide, the slum priest who became his country's first democratically elected president, resigned today under intense pressure from the United States and the threat of an invasion of the capital by armed insurgents, fleeing by jet at dawn under heavy American guard.

President Bush ordered the deployment of the Marines to the country as part of an "interim international force" to restore order. The Marines were expected to arrive in Haiti at any hour. Both France and Canada have agreed to send troops, too. And the United Nations Security Council was scheduled to consider a resolution for a multinational force later today.

Once hailed as the man who could deliver peace and freedom to the world's first black republic, Mr. Aristide saw his presidency crumble as he violently cracked down on political dissent and armed rebels seized Haiti's north this month. The Bush administration shifted its policy this weekend from opposing regime change to strongly urging him to step down.

The chief justice of the Supreme Court, Boniface Alexandre, was sworn in as the head of a transitional government until elections in 2005, following a succession process that is outlined in Haiti's constitution, and Prime Minister Yvon Neptune will retain his post until new elections are held.

In his letter of resignation, read at a news conference this morning by Prime Minister Yvon Neptune, Mr. Aristide said he had chosen to resign to prevent further bloodshed in an armed uprising that has killed as many as 100 people and to ensure that the new government would conform with Haiti's constitution.

"The constitution is the guarantee of life and peace," he wrote in the letter, which was dated Saturday, Feb. 28. "It should not be drowned in the Haitian people's blood. This is why tonight, if it is my resignation that will prevent a bloodbath, I accept to go with the hope that there will be life and not death. Life for everybody, death for nobody by respecting the constitution and have it respected, Haiti will find life and peace."

James Foley, the United States ambassador to Haiti, said that an international force consisting of security, humanitarian and administrative personnel "will rapidly be in Haiti" to help stabilize the country, which has been wracked by violence since early this month. Violent clashes between government supporters and armed militants have left as many as 100 people dead, and widespread looting has left ports, shops and houses in several cities plundered.

In sending the Marines to Haiti, Mr. Bush said: "The government believes it is essential that Haiti have a hopeful future. This is the beginning of a new chapter. I would urge the people of Haiti to reject violence, to give this break from the past a chance to work. And the United States is prepared to help."

About 50 Canadian soldiers provided security at the airport in Port-au-Prince today, assisting a Hercules transport to fly Canadians out of the country to the Dominican Republic. Canadian officials promised today that they would provide further military assistance as part of an international force.

A company of the Second Battalion Royal Canadian Regiment, or about 120 troops, were standing by in Trenton, Ontario, to be dispatched to the country.

"We'll be there when the conditions are right," Foreign Affairs Minister Bill Graham said in a television interview today. "It's important to get some troops in there." Mr. Graham spoke on the telephone with Secretary of State Colin L. Powell today to coordinate assistance.

Mr. Graham also said that Prime Minister Paul Martin will tell the United Nations on Monday that Canada is ready to provide military assistance.

In Paris, French government officials said tonight that France, the first country to demand President Aristide's resignation, was ready to participate in an international peace force in Haiti, but will first await definition of the force's mandate by the United Nations Security Council. They said this force would then complement or replace the United States Marine contingent being dispatched to Haiti.

The officials said that France could send some troops from Europe and use some of those based in its overseas departments. "We're thinking a force of hundreds, not thousands, will be necessary once a government of national unity is in place," one well placed Foreign Ministry official added.

Foreign Minister Dominique de Villepin, who is currently visiting Japan, told the government-owned France 2 television channel that there was also an urgent need for humanitarian aid in Haiti. The officials said that France would dispatch emergency aid to Port-au-Prince in the coming days.

The officials said that, once stability had been restored in Haiti, the international community should also oversee the preparation of new elections and send human rights observers to the country to insure these elections are free.

Although government officials and diplomats urged calm and promised the return of order, little authority was in evidence on the capital's streets today. Chaos and anarchy ruled as news of Mr. Aristide's departure trickled out. After several tense days of expecting a threatened rebel attack, residents lined the streets, anxious for news of Mr. Aristide's fate. Near the airport, where a jet whisked the president away shortly after dawn, a man with a small radio pressed to his ear ran down the street shouting a question to anyone who would listen: "Is it confirmed? Is it confirmed?"

Near the National Palace, opponents of the president began to celebrate, dancing in the streets and weeping. They flashed three fingers in the air, a truncated version of the outstretched palm that the president's supporters use to signify their belief that he should be allowed to finish his five-year term, which was set to end in February 2006.

Mobs of armed young men soon converged on the palace, firing volleys of gunfire, seemingly at random, and thousands of looters poured onto the streets. Rioters torched a Texaco gas station, sending flames shooting skyward into the cauldron of mountains surrounding the city, a black pall belching on the horizon. Mobs began a methodical looting of every shop they could pry open, piling water jugs, fans and groceries onto wheelbarrows and carting them away. Several bullet-riddled bodies were spotted around the city; the total death toll was impossible to ascertain.

Mr. Aristide's dawn departure, long sought by his political opponents, who rejected a power-sharing plan pushed by the United States that diplomats had hoped would defuse the crisis and allow the president to finish his term, was a stunning exit for a man who once seemed poised to deliver his long-suffering nation "peace in the mind, peace in the belly," as his campaign slogan promised.

He was felled by a small but resilient coalition of political opponents and a separate group of armed insurgents who threatened to depose him by force. The rebels, led by veterans of Haiti's army, which was disbanded by Mr. Aristide in retaliation for its role in a 1991 coup that removed him from power, had threatened to attack the capital unless the president left power.

Mr. Aristide was a radical Roman Catholic priest when he rose to prominence in the 1980's as an opponent of military rule and political dictatorship in Haiti. He was expelled from his order for his politics in 1988 and became the leader of a political coalition seeking democracy. Elected president overwhelmingly in 1990, he was overthrown in a violent military coup in 1991 and fled into exile, first to Venezuela, then the United States.

He was returned to power in 1994 by a military invasion led by the United States. Haiti's constitution barred him from succeeding himself as president, but he won a second five-year term in 2000. Over the next three years, his power eroded as political corruption in his government and political anger in the street grew out of control.

Many of his former supporters became his sworn enemies. An armed rebellion led by a gang once loyal to him erupted in Haiti's north on Feb. 5, and several hundred of the rebels quickly seized half the nation and threatened to storm the capital, sparking fear and havoc.

Roughly 100 Haitians have been killed in battles among rebels, police and Aristide supporters this month. Hundreds more have tried to flee in boats bound for Florida; most have been intercepted by the United States Coast Guard and shipped back to Haiti. Mr. Aristide's fall was sudden. Barely 32 hours before he left, in his last address to the nation as president, he said, "I will be at my desk on Monday." American policy toward Mr. Aristide shifted swiftly, too.

In July, Brian Dean Curran, then the United States ambassador here, said, "The United States accepts President Aristide as the constitutional president of Haiti for his term of office ending in 2006."

The Bush administration decided in the past three days, as a senior administration official said Saturday, that "Aristide must go," regardless of his constitutional authority. That message was communicated directly to Mr. Aristide hours before he left this morning. France, Haiti's colonial occupier, also called for the president to step down. In a statement issued Saturday night and authorized by President Bush, the White House blamed Mr. Aristide for "the deep polarization and violent unrest that we are witnessing in Haiti today."

His departure enables a proposed international peacekeeping force to land in Port-au-Prince, secure the capital and enable desperately needed food and economic assistance to flow to Haiti, the poorest nation in the Western hemisphere.

That force would try to stabilize Haiti — a task that could take years — and prevent a fresh flood of desperate refugees trying to reach Florida.

At a news conference here Saturday, the United States ambassador, Mr. Foley, said the United States had learned from its experience in 1994, when it left after just two years.

"I think it is very clear that the effort made 10 years ago did not yield results," Mr. Foley said. "It was not successful and the international community needs to apply those lessons and do better."

Luigi Einaudi, assistant secretary general of the Organization of American States and the group's point man on Haiti, who met often with Mr. Aristide and his opponents, trying to resolve Haiti's deepening political crisis, said in a telephone interview this morning that Mr. Aristide "did what he felt was right for his country, which is in a very polarized state."

"I hope that a succession can be handled constitutionally and peacefully, without excessive turmoil," Mr. Einaudi said. "I'm very concerned that the situation has so already undermined any state authority that things will be difficult in Haiti for a long time."

Robert Fatton Jr., a native of Haiti and chairman of the politics department at the University of Virginia, agreed.

"There are no functioning institutions in Haiti," he said. "Things could very easily unravel. I think we are in for a long crisis. You are going to have a hellish situation without an international peacekeeping force. The armed gangs could go wild. It looks to be a vacuum of power in the short term and that's very dangerous, if there is no center and the country cannot hold."

Paul Farmer, an American doctor who runs a program for AIDS patients in the country's central region, said that however one felt about Aristide, his departure is a sad chapter in a long and sad history.

"This is violent, undemocratic regime change," Dr. Farmer said in an interview from Cambridge, Mass., where he is teaching at Harvard University's Medical School. "There have been more than 30 coups in Haitian history, and I can't see how this one could possibly do anyone any good."

Clifford Krauss provided reporting from Toronto and Alan Riding provided reporting from Paris for this article.