Ivan already has killed 50 people across the Caribbean this week, including 34 in Grenada and five in Jamaica.
Millions more people are in its path, with Ivan projected to go between the Cayman Islands, make a direct hit on Cuba, possibly the Florida Keys, and then either move into the Gulf of Mexico or hit South Florida.
The Hurricane Center said Ivan's widns were measured by a U.S. Air Force reconnaissance plane.
If Ivan hits land in the Caribbean at its current strength, it would be the first Category 5 storm to do so there since Hurricane David devastated the Dominican Republic in 1979, said Rafael Mojica, a meteorologist at the Hurricane Center. Hurricane Mitch was a Category 5 storm in the Caribbean Sea in 1998, but it hit Central America.
Only three Category 5 storms are known to have hit the United States. The last was Hurricane Andrew, which hit South Florida in 1992, killing 43 people.
According to the Hurricane Center, Ivan's "well-defined eye" was moving into position to hit the Caymans sometime Sunday.
Ivan's winds make it "an extremely dangerous Category 5 hurricane on the Saffirs/Simpson hurricane scale," the Hurricane Center said. "Some fluctuations in intensity" are to be expected.
While still a Category 4 storm, Ivan lashed Jamaica with monstrous waves, driving rain and winds nearing 155 mph Saturday, washing away homes and tearing roofs off houses and trees from the ground, but unexpectedly spared the island from a direct hit.
Reporter Ted Scouten of Miami's WFOR-TV, who rode out Ivan in the Jamaican capital of Kingston, told CBS News one gust there was clocked at 189 mph.
In crime-ridden Kingston, sporadic looting and gunfire erupted overnight and continued Saturday morning. Associated Press reporters saw looters carrying boxes of groceries from a smashed storefront.
A 10-year-old girl drowned in Old Harbor, just east of Kingston, and a woman was killed in the capital by a tree that struck here home, said Ronald Jackson of Jamaica's disaster relief agency.
Police said three other people - a man, a woman and a baby - drowned in Clarendon parish, just west of Kingston, according to reports from residents.
Jamaica, an island of 2.6 million known for its tourism, reggae and Blue Mountain coffee, was saved the full brunt of Ivan's fury by an unexpected wobble and lurch to the west overnight.
The change in course could be good news for hurricane-weary Florida, since Ivan may now head into the Gulf of Mexico. Forecasters warned it could still move back to its predicted course and hit the state.
The Cayman government posted a hurricane warning and urged residents to prepare "as for direct impact."
Cuba on Saturday upgraded a hurricane watch to a warning for the threatened western part of the island. Cuban leader Fidel Castro warned residents to brace themselves. "Whatever the hurricane does, we will all work together" to rebuild, he said.
Further north, the Florida Keys were mostly boarded up, deserted by evacuating residents and tourists told days ago to brace for Ivan, which came hard on the heels of Charley and 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."
County officials ordered an evacuation of the entire 100-mile chain of islands, which barely rise out of the water and are extremely vulnerable to storm surge. It was the third evacuation in a month for tourists and first in three years for the chain's 79,000 residents.
In South Florida, long lines reappeared at gas stations and shoppers swarmed home building stores and supermarkets.
On Florida's east coast, hundreds of thousands of Frances victims were still without power in the summer heat. Thousands dealt with overflowing sewers in Palm Beach County. About 767,000 homes and businesses still lacked power Friday.
Throughout the state, memories of the previous storms that killed a total of 50 people in Florida and knocked out power to millions caused residents to line up in droves at gas stations and supermarkets. Officials said gas companies and retailers were trying to avoid the shortages that plagued areas during Charley and Frances.
Florida has not been hit by three hurricanes in a single season since 1964, and this season has been the worst in Florida since 1992. Charley, which hit Aug. 13, and Frances, which hit Sunday, caused up to $20 billion in combined damage in Florida.
Besides the dead in Grenada and Jamaica, Ivan killed five people in Venezuela, one in Tobago, one in Barbados, and four youngsters in the Dominican Republic.
Kingston's telephone service appeared to fail as Ivan passed. Troops on high alert and carrying assault rifles patrolled the darkened city, its electricity cut to protect power plants.
"I'd say we have been spared the worst but we're not out of the woods yet," Jackson said as sheets of rain continued to lash the capital and winds bent palm trees to a 45-degree angle at 8 a.m. local time.
Officials were trying to clear the road to reach the cut-off eastern parish of St. Thomas, believed to be the hardest hit, Jackson said.
In downtown Kingston, 20-foot high trees were uprooted, some flung onto the roofs of cars, and twisted metal roof panels strew the streets. Porcelain tiles that decorated the Jamaica Pegasus Hotel were torn from the facade and smashed to shards.
Howling winds and sheets of horizontal rain crashed around Kingston in the south after Patterson declared a state of emergency and pleaded with the half million people considered in danger - about one in five islanders - to get to shelters. Most refused for fear abandoned homes would be robbed.
In Grenada, where the U.S. State Department was arranging for the evacuations of all Americans who wish to leave.
"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.
In Jamaica earlier, awed onlookers stood transfixed on the seaside Palisadoes Highway near Kingston's airport as 23-foot waves crashed to shore, thrusting rocks and dead tree branches more than 100 feet into the road.
"I've lived here all my life and I've never seen anything like this," said businessman Chester Pinnock, huddled under an umbrella against drenching rain.
In Montego Bay, Jamaica, the Barnett River overflowed its banks, putting some businesses four feet under water and flooding inland roads and farmlands. Drenching rain washed away the main northern coastal road, the A1, just outside Montego Bay.
The British Royal Navy frigate HMS Richmond, which rushed to Grenada's rescue Wednesday, sped to Jamaica along with a supply ship, Commander Mike MacCartain told the BBC.
East of Jamaica, in neighboring Haiti, flooding destroyed at least two houses and damaged a dozen more, but people expressed relief they were spared further catastrophe in a year that has already brought a bloody rebellion and deadly floods.
"First we had a political hurricane, then an economic hurricane and now, with the natural hurricane, we're just glad God saved us," said Jude Vante, 32, an unemployed mason in low-lying Les Cayes, on the southern peninsula.
Ivan became the fourth major hurricane of the Atlantic Season on Sunday. It damaged dozens of homes in Barbados, St. Lucia and St. Vincent Tuesday before making a direct hit on Grenada, which it left a wasteland of flattened houses, twisted metal and splintered wood. It damaged 90 percent of homes there, tossed sailboats to shore and set off looting among some of the 100,000 residents left without electricity, water and telephone service.
Manning, the Trinidadian leader, said Grenada's priorities are establishing security to end looting; recapturing prisoners who escaped from a devastated jail; providing food, potable water, tents, blankets, and materials to rebuild; and restoring communications and electricity.
The American Red Cross disaster unit said Grenada's government has temporarily closed the country to relief shipments to ensure security. The unit's director, Doug Allen, said Grenada needs relief by Sunday to avoid a critical situation.
More than 100 Caribbean soldiers from five countries arrived Thursday to help restore order on the island of 100,000.
On Friday, Trinidadian troops patrolled the marina and shopping area around the downtown Carenage, and police Superintendent Edvin Martin reported only scattered looting.
Up to 75 convicts remained at large after about 150 of the prison's 325 inmates escaped when the storm damaged the prison.
Troops from Barbados and Trinidad guarded Grenada's airport, where dozens of American medical students waited for chartered flights home. There are an estimated 1,000 U.S. citizens on the island, most medical students at St. George's University.