The eye of the hurricane was passing over land in Chetumal on the southern tip of Mexico which borders Belize.
The area of Corozal in northern Belize had been bracing for the full brunt of hurricane conditions.
Rene Villaneuava of BBC Caribbean partner station Love FM in Belize reports that over 10,000 people have been evacuated from areas closest to the storm’s centre.
“Many people are in hurricane shelters awaiting the passage of this dangerous hurricane”, he told BBC Caribbean early on Tuesday morning as his station continued to track the storm.
The area of Corozal on Belize’s north-eastern coast was expected to feel the full brunt of the hurricane’s 160 mile-an-hour winds,.
A curfew has also been imposed in some parts of northern Belize including many of the ‘cayes’ popular with tourists for their coral reefs and other diving attractions.
Further south, in Belize City, officials closed the hospitals and urged people to head inland, saying the town’s shelters were not strong enough to withstand the hurricane.
Cancun and other tourist resorts in Mexico were spared a direct hit as the storm came ashore at 0330 (0830 GMT) about 40 miles (65km) north-east of Chetumal.
The Category Five storm – the highest strength – is moving steadily west with winds of up to 160mph (255km/h).
It is pushing a large surge of water that could flood coastal communities.
The US National Hurricane Center (NHC) in Florida said the eye of the storm had made landfall near the town of Majahual.
Last-minute evacuations took place as the powerful storm was expected to bring a flood surge of up to 18 feet (5.5 metres).
The NHC said the Yucatan Peninsula, Belize as well as Guatemala and northern Honduras could expect heavy rainfall which could cause flash floods and landslides.
Jamaica election
A formal announcement is expected today postponing Jamaica’s general elections originally scheduled for next Monday, August 27.
Jamaicans are now expected to go to the polls on September 3rd as the post Dean clean-up continues.
The hurricane has already claimed at least 11 lives in the eastern Caribbean, but largely spared the low-lying Cayman Islands on Monday.
In Jamaica, it tore off roofs, uprooted trees and downed power lines as the storm’s centre passed just south of the island.
Tourist resorts have been boarded up and offshore oil facilities closed but there is concern for residents of poor communities living in flimsy homes.
Andrea Montalvo of the American Telemundo television network said the storm was wreaking havoc in the Mexican city of Chetumal.
“Inside the hotel it is really bad, every 10 or 15 minutes you can hear windows shattering and people are coming out of their rooms in panic,” she said.
“Some people have been moved to schools on higher ground, but if this is how it is here in this hotel, which is pretty solid, I don’t want to think about how it is there.”
She said the shelters in Chetumal and surrounding areas did not seem well prepared and had holes in the walls and no food or water.
Further south, in Belize City, officials closed the hospitals and urged people to head inland, saying the town’s shelters were not strong enough to withstand the hurricane.
Cancun and other popular Mexican resorts are escaping a “direct hit” but that has not stopped around two-thirds of Cancun’s tourists leaving the area.
Category Five storms are rare – only three have hit the United States since record-keeping began.
Some holidaymakers camped overnight at Cancun’s airport to find a flight while others were turned away.
Police officers have been deployed to prevent looting while residents have boarded up their homes in preparedness for the storms.
Mexico’s state-oil company, Petroleos Mexicanos, has evacuated the remainder of its 14,350 workers and shut down production on the offshore rigs.
The hurricane is moving steadily west at about 20mph (32km/h) and is expected to lose some strength across the Yucatan Peninsula before being re-energised as it hits sea again over the Bay of Campeche on Tuesday night, the NHC said.
It is due to hit Mexico’s coast a second time in the state of Veracruz.
‘A good beating’
Dean has already caused widespread damage as it careered along the south coast of Jamaica.
“I took a journey onto the streets of Kingston and saw huge trees, massive mango trees, coconut trees that have blocked the roads,” said Kathy Barrett from Radio Jamaica.
How hurricanes are tracked
Dean’s path of destruction
“Power lines are down, it really has been – we got a good beating from Hurricane Dean.”
Prime Minister Portia Simpson Miller declared a month-long state of emergency, widening the powers of security forces.
A general election is due to take place on 27 August, but the storm has cast doubt on that date.
In the US, the return of the space shuttle Endeavour was brought forward by a day, to Tuesday in an attempt to beat the hurricane should it eventually reach Texas, where Nasa’s mission control is based.