Storm surges have reached as high as the third storey of some hotels as Hurricane Wilma batters Mexico's popular Cancun resort area.
Even though slightly weakened, it felled trees and tore off roofs as thousands stayed in emergency shelters.
Local governor Felix Gonzales said winds of 140mph (225km/h) had damaged buildings considered hurricane-proof.
At least three people are reported to have died in separate incidents, say state and medical officials.
In the resort town of Playa del Carmen, two people died when a gas tank exploded and west of the Yucatan peninsula, a man died after he was crushed under a large branch.
Wilma is expected to linger over the Yucatan peninsula until early on Monday, said the US National Hurricane Center (NHC).
It was downgraded to a Category-Two storm, packing winds of 110 mph (177km/h) by 1800 GMT.
But the NHC still described Wilma as "really clobbering north-eastern Yucatan".
In Cuba, some 370,000 people have been ordered out of the hurricane's path as it lashes western areas with heavy winds and rain.
And people are fleeing their homes in Florida, which the storm is expected to reach early next week.
Hundreds of thousands of tourists and residents have moved to shelters in Cancun.
Hotel windows on the resort island of Cozumel have been shattered, and Playa del Carmen severely damaged.
There have been some reports of looting and police officers have been standing guard outside some of the larger convenience stores.
"This is the equivalent of having four or five hurricanes of this size pass over one after the other, given the amount of time we have been suffering hurricane-force winds," said Felix Gonzalez Cantu, Governor of Quintana Roo state that includes Cancun.
"Never in the history of Quintana Roo have we had a storm like this.
"The water is crossing over from the sea into the lagoon," he said.
The hurricane has already left Cancun without electricity and with only sporadic telephone service, with hundreds of power and telephone poles torn down.
Mr Gonzales said the storm had caused "great destruction".
With Yucatan airports now closed, tens of thousands of tourists in the area have been moved inland or are now taking cover from the winds.
Conditions in scores of emergency shelters in Cancun and elsewhere are said to be cramped and hot, with no power to run air conditioning.
"I never in my life wanted to live through something like this," cook Guadalupe Santiago told the Associated Press news agency as winds lashed the Cancun hotel where she had taken refuge.
The BBC's Jeremy Cooke in Cancun says the spectacle has remained both dramatic and unchanging for hours, with an increasing amount of debris blown from buildings as they slowly succumb to the storm.
The peninsula and isolated areas of Cuba can expect 25 to 50cm (10 to 20in) of rain by Sunday, with some parts of Cuba hit by up to 100cm (40in), the NHC says.
Huge waves are crashing on to Cuba's westernmost tip, and heavy rains were reported to have cut off several small communities.
In Florida, Governor Jeb Bush has declared a state of emergency, although mandatory evacuations for Florida Keys residents were put back, as Wilma's slow speed means it may not arrive until Monday.
The year 2005 has been one of the most destructive hurricane seasons on record. Wilma is the 12th of the year - a figure equalled only once, in 1969, since record-keeping began in 1851.
By one measure, Wilma was the strongest ever, with the lowest barometric pressure on record in the Atlantic basin.
Thousands of people died in Central America earlier this month in landslides and floods following torrential rains brought by Hurricane Stan.
Mexico: Tourists told to leave high-risk coastal resorts
Cuba: Coastal residents evacuated and some schools closed
US: Visitors and non-residents ordered to leave Florida Keys
Times: All times GMT