Wilma, which left 13 dead in the Caribbean, was expected to nick the northeastern tip of the Yucatan Peninsula before dawn on Friday, then make a beeline for southern Florida, where thousands were being evacuated.
It was downgraded to a Category 4 storm with 230 kph winds after briefly becoming the most intense storm on record in the Americas on Wednesday.
Forecasters at the National Hurricane Center in Miami said the storm was "extremely dangerous." It would be the second hurricane to hit Cancun and its neighbouring resorts this year, following Hurricane Emily in July.
With rains and rough surf already pounding Cancun, officials ordered some 20,000 tourists to leave the row of highrise hotels that line Cancun's famous beachfront, although some ballrooms would be turned into shelters, Cancun Mayor Francisco Antonio Alor said.
He said the city was chartering flights to try to evacuate tourists before the airport closed, probably later Thursday afternoon.
All but emergency officials had been evacuated from the
nearby islands of Isla Mujeres and Holbox.
In Florida, officials in the Keys put off the mandatory evacuation of residents until Friday. Tourists had been told to leave on Wednesday, and the streets were almost empty early Thursday.
Cancun residents brace
themselves for the storm
The massive storm has already whipped through the Caribbean, forcing thousands to leave low-lying areas in a 1000-km swath covering Cuba, Mexico, Belize, Honduras, Jamaica, Haiti and the Cayman Islands.
"People should take this hurricane very seriously," said Scott McClellan, spokesman for US President George Bush.
Honduras, which took a lashing from the storm on Wednesday, escaped major damage. Power was cut to about 20 towns, two Caribbean ports were closed, and four fishermen were reported missing at sea. On Honduras' popular Bay islands, about 500 US and European tourists were moved to safe locations at hotels.
Lines of buses evacuated thousands from hotels while employees boarded up windows. Tourists packed Cancun's airport looking for flights home or to other resorts.
Mark Carara cut his family's vacation short by two days, and tried to get on a standby flight home to Colorado Springs.
"You hear it was the biggest storm on record, and yeah, that was the clincher right there," he said. "It was time for us to go."
Early Thursday, Wilma was centred 285 km southeast of Mexico's Cozumel Island, and was moving northwest at 12 kph. Forecasters said it could strengthen before hitting land.
The storm hit as much of Central America and southern Mexico was still recovering from Hurricane Stan, which left more than 1500 people dead or missing. In the US, Americans were still mourning 1200 Gulf Coast victims of Hurricanes Katrina and Rita.
Wilma may strengthen before
"This is getting very powerful, very threatening," Mexican President Vicente Fox said of Hurricane Wilma.
Hundreds of schools in the Yucatan peninsula were ordered closed on Thursday and Friday, and many will be used as storm shelters.
Florida has seen seven hurricanes hit or pass close by since August 2004, causing more than US$20 billion in estimated damage and killing nearly 150 people.
"People have learned their lesson and know better how to prepare. We're not waiting until the last minute anymore,"
said Andrea Yerger, 48, of Port Charlotte, Florida.
The head of Haiti's civil protection agency, Maria Alta Jean-Baptiste, said that at least 12 people had died in rain and landslides there since Monday. At least 2000 Haitian families had been forced from flooded homes.
In Cuba, civil defence officials said more than 96,000 people had been evacuated from their homes in low lying areas. Some 14,500 students at boarding schools in rural Havana Province surrounding the nation's capital were sent home to their families until after the storm had passed.
Jamaica, where heavy rains have fallen since Sunday, closed almost all schools and 350 people were living in shelters. One man died on Sunday in a rain-swollen river.
Vincente Fox said the hurricane
was 'very threatening'
In the Cayman Islands, schools and most businesses were closed as heavy rains fell intermittently.
In Belize, a nation south of Mexico's Yucatan Peninsula, officials cancelled cruise ship visits and tourists were evacuated from islands offshore.
Wilma's confirmed pressure readings early on Wednesday dropped to 882 millibars, the lowest minimum pressure ever measured in a hurricane in the Americas, according to the hurricane centre. Lower pressure translates into higher wind speed.
Forecasters said Wilma should avoid the central US Gulf coast devastated by Hurricanes Katrina and Rita earlier this year. Those storms killed more than 1200 people and caused billions of dollars in damage.