Hurricane Irma is blasting up the west coast of Florida and is now bearing down on the city of Tampa.
Irma made landfall on Marco Island off Florida's west coast with winds of up to 120mph (192km/h), but has since been downgraded from category three to two.
More than 3.4 million homes in the state are without power, and parts of the city of Miami are under water.
Three storm-related deaths have been reported, as Irma moves northwards.
In an update at 03:00 GMT Monday, the National Hurricane Center (NHC) says "hurricane conditions are continuing across portions of the central Florida peninsula".
Irma currently has maximum sustained winds of 100mph (160km/h), the NHC says, with a wind gust of 96mph reported in Clearwater Beach and one of 91mph recorded in Egmont Channel.
The storm was earlier pummelling the area around the city of Fort Myers.
Irma has already devastated parts of the Caribbean, killing at least 28 people.
Some 6.3 million people in Florida had been told to evacuate.
President Donald Trump has approved a major disaster declaration and emergency federal aid for Florida.
He described the hurricane as a "big monster", praising the federal agencies involved with the storm and saying he would go to the state "very soon".
"We may have been a little bit lucky in that it went on the west, and it may not have been quite as disruptive, but we're going to see, it's going to play out over the next five or six hours," Mr Trump added.
At 05:00 GMT, the centre of the hurricane was about 15 miles (20km) south-west of Lakeland, a town 32 miles north-east of Tampa.
Some 3m people live in the Tampa Bay area. The region has not been hit by a major hurricane since 1921.
Irma is now losing strength, the NHC says, predicting that Irma will become a tropical storm over "far northern Florida or southern Georgia on Monday".
"On the forecast track, the centre of Irma will continue to move over the western Florida peninsula through Monday morning and then into the south-eastern United States late Monday and Tuesday."
The storm made landfall at Marco Island at 15:35 local time (19:35 GMT).
The NHC earlier tweeted that people in the area of Naples and Marco Island should move away from the coast as storm surges of up to 15ft (4.5m) were possible.
There has been some flooding in the centre of Naples but it is believed to be fresh water.