Amateur video of the nighttime accident posted on YouTube showed a large flash as the high-voltage power line caught a popular singer known as Fantom atop the float as it passed near the presidential stand, which was packed with spectators.
Most of the victims were trampled to death in the ensuing panic after the singer struck the power cable, witnesses said.
Fantom, part of the hip-hop band Barikad Crew, was among the injured and was in stable but serious condition in a hospital, according to one of his friends.
Prime Minister Evans Paul held a news conference to announce the cancellation of Carnival and three days of mourning.
"We are telling the people of Haiti that we must be in solidarity," said Mr Paul, who was accompanied by government ministers, artists and first lady Sophia Martelly.
`We are all Haiti.`
President Michel Martelly expressed his "sincerest sympathies" to the victims in a Twitter message and Sophia Martelly visited hospitals that were treating the injured.
They later joined cabinet ministers as well as prominent artists and carnival organisers at a silent parade to remember the dead, with marchers dressed in white carrying candles as a sign of mourning.
"We need to honor these people, to show their families that we are with them," said Theodore "Lolo" Beaubrun of the band Boukman Eksperyans. "It`s terrible: they died in a time of joy unfortunately."
Added Roberto Martino of the group T-Vice: "It is an inexplicable feeling that takes us today. We were happy to party and within 5 seconds, now we are crying."
Haiti`s rambunctious three-day annual street parade coincides with other Mardi Gras celebrations around the world and attracts large nighttime crowds eager to witness competing bands atop highly decorated floats.
At Brazil`s Carnival, three men were electrocuted on Tuesday when they pushed a float toward a parade ground and it struck a high-tension power cable in the Nova Iguacu suburb of Rio de Janeiro, police said.
A sun on the float made of wire touched the power line and sent a fatal electrical charge through the metal frame of the decorated platform, police said. Authorities canceled Carnival festivities in the district after the accident.
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