Hurricane Irma has made landfall in the Florida Keys as a powerful Category 4 storm, the US National Hurricane Centre (USNHC) reports.
The centre of the massive hurricane made landfall on Cudjoe Key in the lower Florida Keys at 9:10am local time (11:10pm AEST).
Irma lashed the area with maximum sustained winds near 215 kph and the USNHC said it was expected to remain a powerful storm as it moved through the Florida Keys and near the state's west coast.
Tens of thousands of people huddling in shelters watched for updates as the storm swung to the west, now potentially sparing Tampa and Miami the catastrophic head-on blow forecasters had been warning about for days.
In the Tampa Bay area, access to all of Pinellas County's barrier islands, including the popular spring break destination of Clearwater Beach, was shut off.
The leading edge of the immense storm bent palm trees and spit rain across South Florida, knocking out power to more than 430,000 homes and businesses.
As the hurricane's eye approached the Florida Keys, Carol Walterson Stroud, 60, and her family were huddled in a third floor apartment at a senior centre in Key West.
"We are good so far," she said in a text message just before 5:30 am. "It's blowing hard."
Ms Stroud said she planned to step outside once the eye of the hurricane passed over. She said she has stood in the eye of a hurricane before and it was "total peace and quiet".
Authorities 'very concerned' about sea surges
Florida Governor Rick Scott warned sea surges might devastate parts of Florida.
He said a powerful sea surge would accompany Hurricane Irma as the storm moves through Florida, and that blast of ocean water could badly damage coastal areas.
"I am very concerned about the west coast," Mr Scott said of Florida's western shoreline that faces the Gulf of Mexico.
"This storm surge is just deadly."
Earlier, Mr Scott had warned residentsin the state's evacuation zones on Saturday that "this is your last chance to make a good decision". About 6.4 million people were told to flee.
But because the storm is 560 to 640 miles wide, the entire Florida peninsula was exposed. Forecasters said the greater Miami area of 6 million people could still get life-threatening hurricane winds and storm surge of 1.2 to 1.8 metres.
The head of the US federal emergency agency said the storm was bringing tornado watches and warnings around the state.
"This is a worst case scenario for Monroe County, the Florida Keys and the west coast of Florida," Brock Long, administrator of the Federal Emergency Management Agency, told the Fox News Sunday.
"Any time you're in that northeast quadrant as the storm is moving forward, that's where the maximum radius winds are that define the intensity of the storm.
'This is going to sneak up on people'
Irma was at one time the most powerful hurricane ever recorded in the open Atlantic with a peak wind speed of 300 kph last week.
It left more than 20 people dead across the Caribbean and as it moved north over the Gulf of Mexico's bathtub-warm water, it was expected to regain strength.
Meteorologists predicted Irma would plow into the Tampa Bay area on Monday morning.
The area has not been struck by a major hurricane since 1921, when its population was about 10,000, National Hurricane Center spokesman Dennis Feltgen said. Now around 3 million people live there.
The latest course also still threatens Naples' mansion and yacht-lined canals, Sun City Centre's retirement homes, and Sanibel Island's shell-filled beaches.
Irma's course change caught many off guard and triggered a major round of last-minute evacuations in the Tampa area. Many businesses had yet to protect windows with plywood or hurricane shutters.
Some locals grumbled about the forecast, even though Florida's west coast had long been included in the zone of probability.
"For five days, we were told it was going to be on the east coast, and then 24 hours before it hits, we're now told it's coming up the west coast," Jeff Beerbohm, a 52-year-old entrepreneur in St. Petersburg, said.
Nearly the entire Florida coastline remained under hurricane watches and warnings, and the latest projections could shift again, sparing or savaging other parts of the state.
Forecasters warned of storm surge as high as 4.5 metres.
"This is going to sneak up on people," Jamie Rhome, the head of the hurricane centre's storm surge unit, said.
Given its mammoth size and strength and its course up the peninsula, it could prove one of the most devastating hurricanes ever to hit Florida, and inflict damage on a scale not seen here in 25 years.
Hurricane Andrew smashed into suburban Miami in 1992, with winds topping 265 kph, damaging or blowing apart over 125,000 homes. The damage in Florida totalled $26 billion, and at least 40 people died.