Hurricane Ivan bore down relentlessly on this lush Caribbean island Thursday evening, and Jamaicans battened down while officials urged preparations and prayer.

Prime Minister P.J. Patterson appealed for calm a day after the storm, one of the most powerful in a decade, killed at least 20 people and ravaged the island of Grenada. The eye of the hurricane, packing sustained winds of 150 miles an hour, was expected to pass directly over this capital during the day Friday.

At 11 p.m. Thursday, Ivan was located 290 miles southeast of Kingston, moving west-northwest at nearly 13 miles per an hour. The National Hurricane Center in Miami forecast that the storm could approach Florida by Monday, but said the margin of error for its future path was wide.

In Key West, Fla., officials ordered the evacuation of thousands of tourists Thursday morning, and authorities were expected to order all 79,000 residents from the Florida Keys on Friday. Portions of Florida are still recovering from the devastation of Hurricanes Charley and Frances in recent weeks.

Ivan's direct hit on Grenada caused destruction "beyond any imagination," Grenadan Prime Minister Keith Mitchell said in comments broadcast on radio stations in Barbados. Officials in Grenada, 1,050 miles southeast of here, said the storm damaged 90 percent of the island's homes, left concrete structures in rubble and destroyed a 17th-century stone prison, sending inmates fleeing.

"We have to prepare for the worst-case scenario," the Jamaican prime minister said in an address Wednesday evening, hoping to persuade citizens, who are used to frequent warnings during hurricane season, to take Ivan seriously. "This is a time that we must demonstrate that we are indeed our brothers' and sisters' keepers," Patterson said. "At this difficult time, let us pray for God's care, his guidance and his blessing."

A headline in the Gleaner newspaper read: "Bracing for Ivan the Terrible." An editorial warned that the storm's effects in Grenada "should compel those Jamaicans who have grown cynical over storm warnings to sit up and take notice."

A sunny afternoon in Kingston gave way to an increasingly ominous, dark and cloudy sky. Stores were reporting runs on water, batteries, buckets and lanterns, and lines were growing at gas stations.

"All we can do is store water, store food, button up our houses and pray that it won't happen," said Fitzroy Francis, 57, who, like many, said he feared that the storm could be as devastating as Hurricane Gilbert, which ripped through Jamaica in September 1988 and caused more than $500 million in damage.

Schools and churches were serving as emergency shelters, and the national blood bank was urging Jamaicans to give blood in anticipation of injuries. Perhaps mindful of the storm's destruction of the prison in Grenada, the government called off-duty correctional officers to report to work immediately.

In Florida, anxiety about the third storm to approach the state in a month was widespread. Shutters and plywood, used for Hurricane Frances, remained on homes from the Keys to the Orlando area. Long gas lines formed Thursday in Miami as residents stocked up.

"People are at wit's end that have been impacted by this, and the people we're asking to respond have some challenges of their own,'' Gov. Jeb Bush told reporters.

Long-range forecasting models did not agree about Ivan's path, adding to the sense of anxiousness. Some projected that the storm would spin toward the state's east coast, others toward its southern tip. Still others showed it headed toward the west coast, placing it on a track to revisit towns hit by Charley, such as Punta Gorda and Port Charlotte.

"We're waiting on nature to tell us which way to go," said Mike Stone of the Florida Division of Emergency Management.

The state is still grappling with major power outages spawned by Frances. More than 1 million customers were without power Thursday afternoon, with the largest outages in Palm Beach County and the jostled communities to the north.

Staff writer Manuel Roig-Franzia in Miami Beach and special correspondent Catharine Skipp in Miami contributed to this report.

Shoppers in Kingston, Jamaica, wait in line to stock up on food and water before Ivan arrives. The storm is expected to hit Jamaica this afternoon.