The death toll elsewhere in the Caribbean rose to 33, with more bodies discovered in devastated Grenada and Venezuela's flooded coastline.
Prime Minister P.J. Patterson declared a public emergency in Jamaica and pleaded with the half million people considered in danger to get to shelters.
"Residents living near coastal areas must evacuate before its too late," he said in a address to the nation. "I cannot stress too strongly that Ivan is a dangerous hurricane.
"What we're experiencing now is only the beginning."
Few have complied with the government order for one in five islanders to flee their homes. Only about 1,200 people had moved into shelters around Kingston by Friday afternoon, said emergency management director Barbara Carby, explaining people fear to leave their property to looters.
Awed onlookers stood transfixed on the seaside Palisadoes Highway near Kingston's airport as 23-feet waves crashed to shore, flinging rocks and dead tree branches more than 100 feet (30 meters) into the road.
"I've lived here all my life and I've never seen anything like this," said businessman Chester Pinnock, in a group protected by raincoats and umbrellas against drenching rain that started Friday morning.
"This is going to be disastrous, we could have hundreds dead. Hurricane Gilbert was a puppy compared to this," he said.
Gilbert killed dozens of Jamaicans and devastated the island when it struck as a Category 3 storm in 1988.
In its wake, Ivan left Grenada a wasteland of flattened houses, twisted metal and splintered wood and sparked a frenzy of looting. Authorities found nine more dead in Grenada — including two foreign yachters — raising Ivan's death toll across the region to 33.
The Category 4 hurricane — out of a top scale of five — packed winds of 145 mph and could strengthen before its core nears Jamaica on Friday night or early Saturday, meteorologists said. They warned of "life-threatening" flash floods and mudslides.
U.S. officials ordered people to evacuate from the Florida Keys after forecasters said the storm — the fourth major hurricane of the Atlantic season — could hit the island chain by Monday after passing the Cayman Islands and crossing Cuba. It was the third evacuation in Florida in a month, following Hurricane Charley and Hurricane Frances.
"I have been through a few hurricanes and I haven't seen people leaving like they are leaving," Key Largo resident Mike Winker told CBS News Correspondent Brian Andrews. "There seems to be a sense of urgency of everyone boarding up and getting out of town."
The storm's current path would take it directly over Kingston, Jamaica's sprawling capital of 1 million people in the southeast, and smash across the island to exit around Montego Bay in the northwest.
The British Royal Navy frigate HMS Richmond, which rushed to Grenada's rescue Wednesday, was speeding to Jamaica along with a supply ship, Commander Mike MacCartain told the BBC.
Friday morning, the storm's leading edge was kicking up heavy rain and winds off Jamaica's eastern tip and as far away as Montego Bay, where stranded tourists crowded the airport and armed private guards began patrolling against looters. Driving rains flooded roads with up to a foot of water.
Disaster agency official Faye Headley said 3,800 people had been evacuated from low-lying areas in Montego Bay, a resort of 35,000 people. "Many people have decided not to leave because they are afraid their property will be stolen," Headley said.
The onslaught was expected to impede the evacuation — of one in five Jamaicans, Power 106 radio reported.
In nearby Haiti on Friday, the storm's leading edge forced piles of sand and water up to knee-high into seaside neighborhoods of Les Cayes, a city of 300,000 on the southwest pensinsula. Hundreds of residents sheltered in schools and churches.
Cuba declared a hurricane watch across the entire island Friday, after its leader, Fidel Castro, went on national television the night before warning residents to brace themselves. "Whatever the hurricane does, we will all work together" to rebuild, he said.
In Grenada, an island of 100,000 people that was the hardest hit by the storm so far, authorities tried to get a handle on the extend of the death and destruction, but efforts were hampered by blocked roads, landslides and a lack of telephone service, said Police Commissioner Fitzroy Bedeau.
At least 22 people were killed in Grenada, Bedeau told the AP, including the drowned yachters, whose nationalities he did not know, people trapped in collapsed homes and a few elderly people who apparently died of shock.
Another officer said rescue crews found bodies of seven adults Thursday, all crushed by fallen trees or roofs, in St. George's wealthy Lance-au-Epines neighborhood. That officer, who spoke on condition of anonymity, said authorities were trying to bury bodies right away because the morgue lacks electricity.
The U.S. Peace Corps sent a boat to an isolated region of the north to check on three of its 23 volunteers who officials have been unable to reach. The three last checked in on Tuesday night and confirmed at the time they were in their designated safe house.
"We have every reason to believe that they are perfectly safe and certainly are accounted for. We just have to make contact with them," said Peace Corps spokeswoman Barbara Daly.
Ivan also killed one person in Tobago, five in Venezuela, one Canadian woman in Barbados, and four youngsters in the Dominican Republic who were swept away by a giant wave Thursday even though the storm was nearly 200 miles away.
"The destruction is worse than I've ever seen," said Michael Steele, a 34-year-old Grenada resident whose home was destroyed. "We're left with nothing."
House after house in the capital of St. George's was shredded by whipping winds. Stadium awnings collapsed, church roofs caved in and many trees snapped. Those left standing were stripped of leaves, giving a brownish tinge to debris-strewn hills overlooking the Caribbean Sea.
"When dogs interfere with garbage bags and strew the contents all over the place — that's what Grenada looks like," Trinidadian leader Patrick Manning said after visiting the island Thursday.
Manning met with Grenada's Prime Minister Keith Mitchell and said on his return home that among Grenada's priorities was bringing security to end looting and recapturing prisoners who were terrorizing already traumatized residents.
More than 100 Caribbean soldiers from five countries arrived Thursday to help restore order on Grenada. Trinidadian troops armed with assault rifles were patrolling parts of Grenada on Friday.
Looting broke out Thursday as hundreds of people, including families with children, smashed storm shutters and shop windows to take televisions and shopping carts of food. Some carried away bed frames and mattresses.
An officer of the elite Special Service Unit said several guns were stolen from his station after officers abandoned it in the storm. He spoke on condition of anonymity.
Mitchell, the Grenadian leader, whose own home was flattened, said Ivan also devastated Grenada's important nutmeg crop and did major damage to the island's 17th century stone prison, allowing convicts to escape.
Mitchell had said they included politicians jailed for killings in a 1983 coup that led the United States to invade, but Manning said Mitchell told him those prisoners, jailed for life, had remained in the ruined building. Nineteen Americans died in the invasion, along with some 45 Grenadians and 24 Cubans.