Hurricane Irma has strengthened to a category four storm as it nears Florida, with maximum sustained winds of 130mph (209km/h).
Hurricane force wind gusts are battering islands in Florida's south, the governor says, with the mainland due to be hit in the coming hours.
Water levels are already rising on the coast of the US state where a huge storm surge is expected.
At least 25 people died when Irma earlier hit several Caribbean islands.
In Florida, 6.3 million people - about 30% of the state's population - had been told to evacuate. But on Saturday, the state governor said it was now too late to leave for anyone remaining.
The National Hurricane Centre has issued warnings against "life-threatening" storm surges in the Florida Keys - a chain of small islands in Florida's south - as well as Tampa Bay and other coastal areas.
More than 200,000 homes in the state have been affected by power outages, with 164,000 outages in Miami Dade county alone, according to utilities company Florida Power & Light.
Irma is predicted to hit the coast on Sunday morning, but the outer bands are already affecting the south of the state and central Miami is being lashed by heavy rain.
The Florida Keys have suffered some minor damage and are expected to bear the brunt of the storm in the coming hours.
"If you're in an evacuation zone, you've got to get to a shelter ... there's not many hours left", Florida Governor Rick Scott warned earlier.
He urged residents at home to "stay indoors and move to an interior room away from windows".
The western Gulf coast is expected to be worst affected, with cities such as Tampa and St Petersburg in the path of the storm.
The Tampa Bay area, with a population of about three million, has not been hit by a major hurricane since 1921.