Rebel leader Guy Philippe drove into Port-au-Prince in a convoy of trucks with about 70 armed supporters.
Crowds cheered the rebels outside the presidential palace of deposed President Jean-Bertrand Aristide.
An advance guard of US Marines and French soldiers has arrived in Haiti to try to restore order as part of a UN-backed international force.
US Defence Secretary Donald Rumsfeld said the US was sending 1,500 to 2,000 troops to serve in the international force, which would number less than 5,000 troops.
The White House dismissed as “nonsense” claims by supporters of Mr Aristide that US Marines had forced him to flee Haiti.
Commenting on the US deployment, US Secretary of State Colin Powell said: “I don’t think there will be a great deal of fighting, but they have to be prepared for that”.
The BBC’s Stephen Gibbs, in Port-au-Prince, says a carnival atmosphere greeted the rebels’ arrival.
Mr Philippe’s first stop was a police station in the capital. He says he wants to work with Haiti’s police force and the international troops to restore security.
US Marines have already set up a command centre in the airport’s VIP lounge.
The first groups of about 50 French soldiers, the US Marines and Canadian forces already on the ground will be followed by reinforcements in a mission backed by the United Nations.
Several hundred French soldiers were expected to arrive in Haiti by Monday night, and Paris has also dispatched a platoon of police officers from a special riot control unit.
A United Nations “assessment team” is expected to leave New York for Haiti within days, charged with preparing the way for UN peacekeepers.
Our correspondent says any amateur soldiers roaming the capital’s lawless streets are likely to capitulate when faced with professionals from an international force.
Violence flared in the city after Mr Aristide fled the country, with some of the widespread looting and destruction apparently a frustrated response to the ceding of power.
Mr Aristide, his wife and three children arrived in the Central African Republic on Monday. It is not yet clear whether they will stay there or seek asylum in a third country.
Canadian special forces are also at Port-au-Prince airport, where they are helping Canadian nationals who wish to leave Haiti.
US defence officials say the US will take initial command of the multinational force, though that could change once the full UN operation begins.
The UN Security Council unanimously authorised a force to stay in Haiti for up to three months to restore security and stability on Sunday night.
It is expected to be followed by a UN stabilisation force, probably including civilian police, which has no time limit.
UN Secretary General Kofi Annan said the international community needed to make a long-term commitment to help Haiti “over the long haul”.
Calls for peace
All sides in the Haitian conflict have also called for peace.
Guy Philippe, who had been massing his men for an assault on the capital, welcomed the foreign intervention, said: “We just want peace.”
Interim leader Boniface Alexandre warned that the future would be difficult.
“Haiti is in crisis… It needs all its sons and daughters. No-one should take justice into their own hands,” he urged.
Mr Aristide issued a statement from his temporary shelter in the Central African Republic, criticising the rebels who prompted the crisis by seizing towns and cities across the north in a month-long uprising.
“In overthrowing me, they cut down the tree of peace,” he was quoted as saying by the Associated Press news agency.
“But it will grow again, because the roots are well-planted.”
The violent protests stemmed from disputed elections in 2000, which the opposition says were rigged.