One of the most powerful hurricanes in Jamaica's history is pounding the island with heavy rains and high winds.
Uprooted trees are just one sign of the hurricane's devastating effect
Waves around two-storeys high were reported on its eastern shores as Hurricane Ivan ripped up trees and roofs and left two people dead.
Prime Minister PJ Patterson declared a state of emergency and the national electricity grid was shut down leading to a black-out across the island.
Troops are patrolling the streets, but there have been reports of looting.
Hurricane Ivan has already left a massive trail of damage in the Caribbean, killing at least 27 people, mostly in Grenada.
Winds have increased in the last few hours to 155mph (250km/h). Forecasters warn it is now an extremely dangerous category four - on a scale of five - and could well intensify.
Forecasters say the course of the storm has changed, veering away from the densely-populated capital, Kingston, towards the western coast.
At 1500 GMT the eye of the hurricane - dubbed Ivan the Terrible by Jamaicans - was about 30 miles (50km) southwest of the western tip of Jamaica, the Miami-based National Hurricane Center said.
As dawn broke, Jamaicans were able to see the extent of the devastation.
Trees and roofs are flying around, power lines are down and the city is without light, Kingston resident Ouida Ridguard told the BBC.
"The rains have been torrential. The sound of the wind is frightening and it is quite devastating," she said.
"This is definitely the worst thing I have experienced in my lifetime - and I have been through two major hurricanes," she added.
The driving rain and intense gusts of howling wind began on Friday afternoon.
Half a million people in low-lying areas - a fifth of the population - were urged to evacuate homes and head for shelters.
Although many of the 1,000 shelters had filled up, some residents were loath to leave their homes for fear of looting, our correspondent says.
Correspondents said heavy downpours caused flash flooding, forcing many people to flee their homes, and there are threats of landslides.
The electricity grid was shut down for safety reasons, and only a skeleton service was in operation.
There is no water coming through the taps in houses, says the BBC's Nina Robinson, who is also in Kingston.
'Ivan the Terrible'
Hurricane Ivan threatens to be the worst natural disaster to hit Jamaica for 50 years.
The Cayman Islands are also in its path.
Cuba is facing hurricane warnings, and the south-eastern United States is also under threat.
A mandatory evacuation has been imposed in Florida Keys with more than 60,000 people - already storm-weary after two major hurricanes in the last month - asked to leave their homes.
So far the place hardest hit by Hurricane Ivan has been the tiny island of Grenada.
The first deliveries of aid supplies arrived on the island but the International Red Cross said more than two-thirds of Grenada's 95,000 inhabitants were now homeless. It has called for more donations.
BBC FM in Jamaica will be off air until Hurricane Ivan has passed. Listeners can follow BBC World Service on shortwave frequencies 6195 and 15190 between 0300-1000 local time and on 5975 between 1600-2000 local time.
PREDICTED PATH OF HURRICANE IVAN