Newly born Hurricane Tomas swept through a cluster of eastern Caribbean islands on Sunday, tearing off roofs, damaging houses and downing power lines.
Authorities in St Vincent were trying to confirm reports that three people died, including two men who might have been blown off a roof, said Jimmy Prince, emergency management spokesman.
Fierce winds tore roofs from nearly 100 homes and more than 400 people sought emergency shelter as the island plunged into darkness, he said.
"Many of them are workers who were unable to get off Mustique," he said, referring to a tiny island just south of St Vincent.
In St Lucia, winds ripped off the roof of a hospital, a school, a stadium and toppled a large concrete cross from the roof of a century-old church, government officials said.
Heavy rains also unleashed a landslide that blocked a main highway linking the capital to the island's southern region.
Prime Minister Stephenson King said that he was still stranded in Barbados on an official trip and apologised to people on an island that reported a complete blackout.
"It hurts me to know that I am not around to give courage, strength and guidance at a time when we all must bond together and give support to each other," he said in a statement.
The government ordered two airports and all businesses closed and people called radio stations to admonish parents who were letting children play in the streets, where trees and power lines were falling.
"This is no joke," said calypso singer Nintus, one of the callers.
Organisers of the island's biggest Creole festival called off the event due to the storm, disappointing both would-be revellers and dozens of vendors who travelled to the capital to sell vegetables, fruits and other provisions.
"All my preparations have gone down the drain," said vendor Theckla Darius, from the rural community of Fond Assau. "It's been a lot of effort for nothing."
St Vincent and Martinique, where at least 20,000 people were without power, streets flooded and tree branches were down. A cruise ship carrying nearly 2,000 tourists docked instead in Dominica.
Tomas had already knocked down power lines and damaged houses in Barbados as a tropical storm.