“Everyone in the Florida Keys ... It is time to hunker down,” the National Weather Service warned before the hurricane rammed into the tropical islands, which are popular for fishing, boating and scuba diving.

“The worst winds are yet to come.”

For those still at home, it was too late to flee the wrath of one of the most powerful hurricanes ever to hit storm-prone Florida, after cutting a path of devastation across the Caribbean.

In Key West, police opened a “shelter of last resort” for those who had ignored evacuation orders.

The cities of Naples, Fort Myers and the densely populated peninsulas of Tampa Bay on Florida’s west coast were in the cross hairs of the historic storm, which was churning slowly northwest at eight miles per hour.

“It’s going to be horrible,” Florida Governor Rick Scott said of Irma on NBC television. “Now we have to hunker down and watch out for each other.”

More than 430,000 homes and businesses were already without power, mainly in southern Florida, according to utility company Florida Power and Light, which said it had “safely shut down” one of two nuclear reactors at its Turkey Point power plant.

Scott said President Donald Trump had pledged to provide whatever resources needed to rebuild after Irma’s impact.

At least 25 people have been killed since Irma began its march through the Caribbean, smashing through a string of islands from tiny Barbuda on Wednesday, to the tropical paradises of St Barts and St Martin, the US Virgin Islands, Puerto Rico, the Dominican Republic, Haiti and the Turks and Caicos.

Terrified Cubans who rode out Irma in coastal towns – after it made landfall on Friday as a maximum-strength category 5 storm on the Camaguey archipelago – reported “deafening” winds, uprooted trees and power lines, and blown rooftops.

There were no immediate reports of casualties, but it caused “significant damage”. AFP journalists reported seeing enormous waves lashing the Malecon, Havana’s emblematic seafront, causing seawaters to penetrate deep into the capital.

Irma smashed into the Keys exactly 57 years after Hurricane Donna struck the same area in 1960, destroying nearly 75 per cent of the island chain’s buildings.

The weather service urged Floridians to keep their shoes on, take shelter in interior rooms – far from windows – and use helmets, mattresses, pillows or blankets for protection.

At North Collier Regional Park, a shelter outside Naples, anxious evacuees prayed their loved ones would remain safe.

Viviana Sierra, who sought refuge with her parents, brother and pet dog, was sanguine about the prospect of finding her home and belongings destroyed.

“You can replace material things but your life is very important so I think it’s better that we stay here,” she said.

Irma was expected to move along or near Florida’s southwest Gulf coast throughout Sunday and into Monday. But the hurricane is so wide that authorities were bracing for destructive storm surges on both coasts of Florida and the Keys.

The NHC also warned of tornado risks, with the greatest threat lying in areas east of the storm’s path.

Shopping centres, fast food restaurants and retail giants were closed for business on both of Florida’s coasts.

Florida Highway Patrol said two people had died in a head-on crash in Hardee County, southeast of Tampa, where tornado warnings had been issued.

Agency spokesman Greg Bueno said it was not immediately clear what role the weather may have played in the deaths, but troopers were investigating the accident.

Warning that Irma would be more powerful than Hurricane Andrew, which killed 65 people in 1992, Florida’s governor said all 20.6 million of the state’s inhabitants should prepare to flee. But some were determined to ride out the storm.

Scott Abraham, who lives on the fifth floor of a beachfront block of flats in Miami Beach, planned to ignore evacuation orders and stay with his wife and two kids.

“If I lived in a house I would have left, but if it gets flooded here it’s going to take a week at least to come back. I don’t want that,” he said.

In a welcome reprieve for the Caribbean, a second monster storm, Hurricane Jose, which had been heading toward the same string of islands devastated by Irma, began to gradually weaken and shift course towards the north.

The deteriorating weather had grounded aircraft and prevented boats from getting relief to hard-hit islands. The US military was mobilising thousands of troops and deploying several large ships to aid with evacuations and humanitarian relief.