A high-voltage cable fell on a float during a Carnival parade in Port-au-Prince early Tuesday, killing at least 16 people as thousands watched, officials said.
Another 78 people were injured in the accident, which occurred shortly before 3:00 am (0800 GMT), throwing Haiti into mourning on the second day of Carnival festivities, ordinarily a joyful high point of the year in this impoverished Caribbean country.
Family members besieged a main hospital complex in Port-au-Prince to find their loved ones as doctors struggled to treat the injured.
Authorities cancelled the third and last day of the Carnival celebrations on Tuesday, and declared three days of mourning.
Prime Minister Evans Paul called on Haitians to pay homage to the dead later Tuesday by dressing in white and marching silently at the Champ de Mars, the parade ground where the accident occurred.
The float — carrying a popular rap group called the Barikad Crew — struck a power line overhead as it made its way through the Champ de Mars.
The group's star singer, who goes by the name "Fantom," was struck directly by the fallen cable and is in critical condition, the website Haiti Press Network said.
Many of the dead and injured were dancers and musicians on the float.
Thousands of people were watching the parade as the disaster struck.
"Sharing in the grief of the families affected by this drama, the government has decided to suspend all Carnival festivities across the country," the prime minister's office said in a statement.
The period of national mourning would formally begin Wednesday, with flags to be flown at half-mast on all public buildings.
National funerals were to take place on Saturday.
The area where the accident occurred has been a focal point of Haiti's turbulent modern history — a witness to dictatorships, revolutions and in 2010 a devastating 7.0 magnitude earthquake that killed more than 300,000 people.
The festivities had begun on Sunday with the theme "Nou Tout Se Ayiti" — Creole for "We are all Haiti."
A doctor contacted by AFP said there were scenes of panic at medical centers where the dead and injured were taken.
About 80 to 100 people, some of them weeping, gathered at the morgue of the Hopital General to see if family members had been brought there.
President Michel Martelly extended his "sincere condolences" to the victims in a message on his Twitter account.
Communications Minister Rotchild Francois said the government "stands ready to assist everyone affected by this tragedy."
"The government has made all the arrangements to organize national funerals for the victims of the accident," he said on Twitter.