Rene Preval has been sworn in as Haiti's first democratically-elected president since his predecessor was ousted in an armed revolt in 2004.
Rene Preval has promised ro reconcile divisions
Mr Preval is an ex-ally of Jean-Bertrand Aristide, the former president overthrown two years ago.
Curbing lawlessness and reviving impoverished Haiti's economy will be his priorities, correspondents say.
Hours before Mr Preval was sworn in, a riot broke out at a major prison in the Haitian capital, Port-au-Prince.
Gunshots were heard from the compound and prisoners were seen on the roof waving banners and apparently bloodstained clothing.
UN troops and police had reportedly surrounded the jail.
UN troops deployed since Mr Aristide's overthrow have been unable to restore order in a capital still ruled by armed gangs, many of them loyal to the former leader.
Mr Preval has promised to reconcile divisions and mend the economy.
He has also said there is no constitutional barrier to allowing Mr Aristide, currently in exile in South Africa, to return to the country.
Delegates from some 40 countries attended the swearing-in ceremony in Port-au-Prince.