US President George Bush says he has deployed US Marines to be the "leading element" in an international force to help stabilise Haiti.
Intense international pressure sealed Aristide's fate
Early on Sunday, Haiti's President Jean-Bertrand Aristide flew out of the country in an unmarked jet, following a three-week rebellion against him.
There have been celebrations in some parts, but the capital Port-au-Prince is in the grip of near-anarchy.
The UN Security Council is now meeting to authorise an international force.
Mr Bush said the vanguard of marines would help "bring order and stability to Haiti".
He urged Haitians to "reject violence to give this break from the past a chance to work".
The first US Marines are expected in Haiti shortly.
More than 100 French troops are also en route to Haiti and Canada has promised to assist.
The UN Security Council started a meeting in New York on Sunday evening to discuss a draft resolution authorising a multinational force, prepared by members of an informal "Friends of Haiti" group - including the US, France, Canada and Caribbean states.
Rebel leader Guy Philippe, who had been massing his men for an assault on the capital, welcomed the foreign intervention, saying the time for fighting was over.
There was joy in rebel-held areas as the news spread
"We just want peace," he told the BBC.
He said he was making his way to Port-au-Prince, where his fighters would help impose security, and he denied that he had any political ambitions "for now".
Fury and joy
The elderly head of Haiti's Supreme Court, Boniface Alexandre, has been sworn in as caretaker president, as stipulated by the constitution.
Mr Aristide's destination and current whereabouts are unknown. The US National Security Adviser Condoleezza Rice reportedly said he was in a "third" country - implying he is not in the United States.
Lawlessness continues in the capital, with looting, hijackings, petrol stations torched and hospitals robbed.
At least three people are thought to have been killed.
While some gangs continue to say they are loyal to Mr Aristide, other armed men are now coming out to declare allegiance to Mr Philippe.
In rebel-held Cap Haitien, people danced in the streets on hearing the news of Mr Aristide's departure.
In a statement, Mr Aristide said: "The constitution should not drown in the blood of the Haitian people... If my resignation is to prevent bloodshed, I accept to leave."
Both the US and France had called on the president to step down.
It has emerged that Mr Aristide was escorted to the plane in which he made his dawn escape by US Marines already in the country.
US State Department spokesman Richard Boucher said: "At President Aristide's request, the United States facilitated his safe departure from Haiti."
THE ARISTIDE YEARS
1990: Haiti's first democratically elected president
1991: Overthrown in military coup; exiled to US
1994: Reinstated; forbidden from standing for second consecutive term
2000: Wins contested elections
2004 - Jan: Haiti celebrates 200 years of independence, amid growing political protests
Early Feb: Exiled rebel leaders cross back into Haiti; capture north of country
Feb 29: Aristide leaves Haiti
The White House had been increasingly critical of Mr Aristide, which may have been the final straw for him, the BBC's Branwen Jeffreys reports from Washington.
The escalating violent protests stemmed from disputed elections in 2000, which the opposition says were rigged.
In the past three weeks rebels have taken control of much of the country; and recently law and order broke down completely in the capital.
It is the second time Mr Aristide has been forced into exile. He was ousted in 1991 in a coup within months of becoming Haiti's first democratically elected leader
He was restored to power three years later by a US-led military intervention.