The accident occurred as thousands of people filled the streets of Port-au-Prince for the raucous annual celebration. Video from the scene shows sparks coursing from the wire after the man apparently tried to move it so the float could pass underneath.
Nadia Lochard, a co-ordinator for the Department of Civil Protection, said at least 20 people were killed and 46 injured.
Joel Desire, a doctor at General Hospital, said most of those killed appeared to have been trampled to death as the crowd surged away from the float.
The float, one of 16 in the carnival parade, was sponsored by Haitian hip-hop group Barikad Crew. Some of the victims were instantly electrocuted in a flash of sparks, according to witnesses. Others managed to jump off, causing panic in the crowd.
“I saw the wire falling and sparks and I started running for my life,” said Natacha Saint Fleur, a 22-year-old who was near the float.
Hundreds of people thronged the General Hospital, where many of the victims were taken, some carrying victims and others searching for family members.
Communications minister Rothchild Francis said the government was working to assist victims.
First lady Sophia Martelly visited some of the injured at the General Hospital but left without speaking to reporters.
Three people were also killed yesterday in a similar incident near Rio de Janeiro, Brazil.