Haiti's President Jean-Bertrand Aristide has gone into exile after a three-week rebellion against him, saying he wanted to avoid bloodshed.
Aristide succumbed to increasingly blunt international criticism
Celebrations broke out in rebel-held areas, but angry supporters set fires in the capital, and gunfire rang out.
International military forces would be rapidly deployed to restore security in Haiti, said the US ambassador.
The head of Haiti's Supreme Court says he is assuming power as stipulated by the constitution.
Mr Aristide's plane reportedly landed for a refuelling stop on the island of Antigua, but his final destination is not known.
Fury and joy
Hundreds of people converged on the presidential palace in the capital, Port-au-Prince and a pall of smoke is hanging over the city after at least one petrol station was set on fire.
Looting was reported in surrounding streets.
There was joy in rebel-held areas as the news spread
Members of Mr Aristide's government are said to be flocking to the Dominican Republic Embassy, looking for a way to escape the anarchy in the capital.
The houses of two senior government members have already been targeted by looters.
The BBC's Claire Marshall, in Port-au-Prince, says many people fear the vacuum left by Mr Aristide's departure, when there is no obvious replacement.
But in rebel-held Cap Haiten, people were dancing in the streets at 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."
The chief justice of the Supreme Court, Boniface Alexandre, says he will take over as caretaker president under the rules of the constitution.
US ambassador to Haiti James Foley said an international force, including US soldiers, would arrive in Haiti shortly.
On Friday, three US navy vessels - which could carry 2,000 marines - were placed on standby to go to Haiti.
Both the US and France had called on the president to step down.
Mr Foley said Mr Aristide had "made a decision for the good of the Haitian people".
Events unfolded quickly on Sunday when an unmarked jet carrying Mr Aristide and his security chief left Port-au-Prince airport.
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
Feb 10: (approx) Exiled rebel leaders cross back into Haiti; capture north of country
Feb 29: Aristide leaves Haiti
It later emerged that some of about 50 US Marines guarding the American embassy had escorted him to the plane.
US state department spokesman Richard Boucher said: "At President Aristide's request, the United States facilitated
his safe departure from Haiti."
The White House had made increasingly blunt criticisms of Mr Aristide in the last day, which may have been the final straw for him, says our correspondent in Washington, Branwen Jeffreys.
The unrest which has gathered pace in recent weeks stems from disputed elections in 2000, which the opposition says were rigged in Mr Aristide's favour.
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.
On Saturday, Haitian rebel leader Guy Philippe said he had decided to hold off an attack on the besieged Haitian capital for "a day or two".
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.