The centre of Hurricane Irma has hit mainland Florida, just south of Naples, amid warnings of storm surges as high as 15ft (4.5m).
Irma made landfall on Marco Island off Florida's west coast with winds of up to 192km/h but has since been downgraded to a category 2 storm.
Irma has already devastated parts of the Caribbean, killing at least 24 people and 3 in Florida.
Some 6.3 million people in Florida had been told to evacuate.
U.S President Donald Trump has approved a major disaster declaration and emergency federal aid for Florida.
The US National Hurricane Center tweeted that people in the area of Naples and Marco Island should move away from the coast as storm surges of up to 15ft were possible.
There has been some flooding in the centre of Naples but it is believed to be fresh water.
There is dead calm and people have been out on the streets, but the storm surge is expected to come later.
The storm is now heading for the Tampa Bay area, with a population of about three million.
The region has not been hit by a major hurricane since 1921.
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