The eye of Hurricane Irma has hit Florida's southern islands as a category four storm, forecasters say.
The storm is lashing the low-lying Keys with winds reaching 130mph (209km/h).
Florida Governor Rick Scott said he was "very concerned" about the state's western Gulf Coast, where the storm is expected to head next.
More than 6.3 million people in Florida were told to evacuate, with warnings of a "life-threatening" storm surge. More than a million homes are without power.
Irma has already devastated parts of the Caribbean with at least 25 deaths.
Extreme winds, around the eye of the hurricane, are expected to last for the next two hours in the Lower Florida Keys area, which includes Key West.
All residents had been ordered to leave. There are reports of storm surges on the coral cay islands, most of which are only a few feet above sea level. Some surges could reach 15ft (4.6m).
One official had warned staying on the islands would be "almost like suicide".
Media reports say a man was killed on Saturday in the Keys when his trucked crashed into a tree as the storm gathered pace.
As the eye of the storm is expected to move north to mainland Florida, more than a million homes in the state are reported to be without power and some 50,000 people have taken refuge in shelters.
Cities such as Tampa and St Petersburg lie 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.
Governor Scott told NBC's Today Show that though authorities had prepared all week for the arrival of Irma, the prospect of such a large storm surge was "really scary".
Irma is the most powerful Atlantic storm in a decade, and has already caused widespread destruction on several Caribbean islands:
Another storm, Jose, further out in the Atlantic behind Irma, is a category four hurricane, with winds of up to 130mph.
It initially followed a similar path to Irma and had threatened several islands already hit by its predecessor, but it has now tracked harmlessly to the north.
Barbuda, whose residents had already left the island as Jose approached, was spared, as were St Martin and St Barthelemy.
Hurricane Katia, in the Gulf of Mexico, a category one storm with winds of up to 75mph, made landfall on the Mexican Gulf coast in the state of Veracruz late on Friday before weakening to a tropical depression.