In St. Lucia, fierce winds ripped off the roof of a hospital and a school in the eastern village of Dennery, legislator Edmund Estephane told state radio.
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.
Organizers of the island's biggest Creole festival called off the event due to the storm, disappointing both would-be revelers and dozens of vendors who traveled 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."
Also under hurricane warning were 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.
The U.S. National Hurricane Center in Miami said Tomas was a Category 1 storm with winds of 75 mph (120 kph) and was centered about 35 miles (55 kilometers) southwest of St. Lucia on Saturday afternoon. It was moving west-northwest near 10 mph (17 kph). Tomas, the Atlantic season's 12th hurricane, is expected to drop up to 6 inches (15 centimeters) of rain in the region.
Forecasters said it could become a Category 2 storm Monday evening and possibly reach Category 3 by midweek, with winds around 115 mph (184 kph).
It was forecast to head toward Jamaica and could unleash heavy rains in southern portions of the Dominican Republic and Haiti, which is struggling to recover from a devastating earthquake and cope with a recent cholera outbreak.
Haiti issued an orange storm alert, the second highest level. Authorities warned southern and western regions - including the quake-ravaged capital of Port-au-Prince, where an estimated 1.3 million people are living in tent camps - to be on guard for high winds, thunderstorms and possible flooding.
But with few usable storm shelters and no feasible evacuation plan, residents will largely be on their own.
Tropical storm warnings were issued for Dominica, Tobago and Grenada, where the airport closed and gas stations sold out of fuel.
Another tropical storm, Shary, headed into the open Atlantic after missing Bermuda.
Associated Press writer Rodolphe Lamy contributed from Fort-de-France, Martinique.