The tiny low-lying Cayman Islands are taking a battering from 155 mph (250km/h) winds as one of the Caribbean's worst ever hurricanes passes by.
Cubans are taking no chances as Ivan nears
Residents who have managed to get messages out of the British dependency speak of terrifying scenes of roofs being ripped off and homes collapsing.
Much of the main island, Grand Cayman, is said to be under water, including the international airport's runway.
Cuba's western tip is braced to be hit on Monday morning local time.
"It's as bad as it can possibly get," Justin Uzzell, 35, said by telephone from the fifth-floor of a Grand Cayman office building.
"It's a horizontal blizzard. The air is just foam," he was quoted by the Associated Press as saying.
The BBC's Stephen Gibbs in Havana says the destruction in the Cayman Islands is a final warning to Cuba of what to expect.
Many of the best-constructed buildings on the Cayman Islands - famed as an affluent, off-shore banking centre - appear to have been unable to withstand the power of Ivan, he adds.
Hurricane Ivan is the sixth-strongest storm to ever hit the Atlantic basin, the National Hurricane Center has said.
It has led to the deaths of more than 40 people in the Caribbean, and the death toll is expected to rise.
State of emergency
Although the storm was downgraded to a category four - from a top level of five - as it hit the Cayman Islands, the destruction looks set to be extensive.
Many of the 45,000 residents were hiding in homes and shelters as the hurricane unleashed ferocious winds, rain and waves of up to 20ft (six metres) while cutting off power lines, uprooting trees and scattering debris.
At least one of the shelters was said to have been damaged.
"We know there is damage and it is severe", Wes Emanuel
of the Cayman Government Information Service told the Associated Press news agency.
A state of emergency has been declared, and water supplies turned off for fear of contamination during the storm.
Most residents on the smaller islands of Little Cayman and Cayman Brac were evacuated to the main island of Grand Cayman, the Associated Press reported.
Cuba's government has evacuated 1.3 million people from areas expected to be hardest hit, and ordered people in Havana's most vulnerable buildings to move to safety.
State media is broadcasting non-stop information on the likely path of the hurricane, which looks set to cross the western tip of the island early on Monday, the BBC's Stephen Gibbs in Havana reported.
Cuban President Fidel Castro said the government was doing everything possible to prepare for Ivan's arrival.
Mexico has also issued a hurricane watch for its north-eastern Yucatan Peninsula. Mandatory evacuations have been under way in Florida Keys.
Meanwhile, a massive clean-up operation is under way in Jamaica after Ivan - and intense rain afterwards - wrecked the island.
The authorities in Jamaica have appealed for international help after being hit by Ivan on Friday night when it was a category-five hurricane.
"We can of course within our own resources meet some of the needs but of course we are expecting and hoping and depending on international support whether from the UN family or donor agencies," Jamaica's Deputy High Commissioner in London, Sharon Saunders told the BBC.
Flooding continues to be the main problem in Jamaica.
Homes and roads were swept away in flooding caused by heavy rain and huge waves up to 23ft (seven metres) high.
Of the half a million people in the exposed eastern shores who had been urged to move into shelters, many ignored the advice fearing their homes might be looted.
There is no electricity and many places are without water.
PREDICTED PATH OF HURRICANE IVAN