Hurricane Ian has ploughed into Florida's Gulf Coast with catastrophic force, assaulting the state with howling winds, torrential rain and a treacherous surge of ocean surf.
About 2.5 million people were ordered to evacuate south-west Florida.
As the storm hit, streets were swamped with water as winds smashed trees along the coast.
A storm surge is forecast to reach up to six metres, threatening catastrophic flooding across a wide area.
Naples city Mayor Teresa Heitmann said authorities were expecting severe damage.
"We also are very concerned about our utilities. We want to make sure our water source is protected," she said.
"There is a curfew in place now because so many people wanted to come out and see the action at the beach, which has put many people at risk."
No deaths were reported in the United States from Ian by late Wednesday (local time).
But a boat carrying Cuban migrants sank on Wednesday in stormy weather east of Key West.
Ian is one of the most powerful storms on record to hit the United States, with sustained winds of up to 241 kilometres per hour, according to the US National Hurricane Center (NHC).
The storm's wind speeds put it just shy of a category five designation on the Saffir-Simpson scale, the most severe classification for storms.
Within eight hours of landfall, Ian was downgraded to a category one storm with winds sustaining 150kph according to the NHC.
It continued to unleash drenching rains as it crept farther inland, threatening to trigger extensive additional flooding.
"This storm is doing a number on the state of Florida," Governor Ron DeSantis said.
Mr DeSantis has called on US President Joe Biden to approve a major federal disaster declaration to provide emergency aid to the entire state.
Before making its way through the Gulf of Mexico to Florida, Hurricane Ian tore into western Cuba as a major hurricane on Tuesday, killing two people and bringing down the country’s electrical grid.
Power outages across Florida
More than 2 million Florida homes and businesses were left without electricity, according to the PowerOutage.us site.
Nearly every home and business in three counties were without power.
Cuban officials said they had begun to restore some power on Wednesday after Hurricane Ian knocked out electricity to the entire country when it hit the island's western tip on Tuesday.
Authorities said it was now too late to get out of harm's way and urged anyone still in the storm's path to hunker down.
In Sarasota County, officials warned that emergency vehicles would not respond to calls for help until it was safe to be on the road.
"Most people heeded the warnings of doing the evacuations in those very sensitive locations, but not everyone may have done that," Mr DeSantis said.
"There will be debris in the air and flooding powerful enough to move cars around, so please do not be outside.
"This is this is a really, really significant storm. It will be one of the storms people always remember."
Mr DeSantis said while the storm surge had likely peaked, damage would likely be inflicted across the state.
In Venice, a coastal city of nearly 24,000 residents halfway between Tampa and Fort Myers, rain and wind were already intense before noon.
Doug Coe was one of the town's residents who chose to ignore warnings and stay put.
As he walked through rainfall on Wednesday morning to see how a friend's home was weathering the storm, Mr Coe admitted to never experiencing a storm of this magnitude, but he was unfazed by the prospects of it ravaging his neighbourhood.
"You have to be vigilant because you never know what's going to happen with it," he said.
"I'm staying vigilant, but trying not to worry."
Many streets were flooded, and the steady gale bent palm trees at 45-degree angles and shredded billboards and road signs.
Forecasters said Ian would unleash storm surges — wind-driven coastal flooding — of up to 3.7 metres along with intense thunderstorms and possible tornadoes.
"This is a devastating storm," Ken Graham, director of the National Weather Service, said.
"This is a storm that we will talk about for many years to come, an historic event."
Water inundates hospital, roof 'blows off'
The hurricane has swamped a Florida hospital, with the storm surge flooding its lower level emergency room as fierce winds tore part of its fourth floor roof from its intensive care unit, according to a doctor.
Doctor Birgit Bodine said they did not anticipate that "the roof would blow off on the fourth floor".
She said ICU patients had to be moved, with two of the hospital's four floors sustaining damage.
Dr Bodine said patients were understanding and they were bracing to treat people impacted by the hurricane.
"For us, as much as everything is terrible and we're exhausted … as long as our patients do OK and nobody ends up dying or having a bad outcome, that's what matters," she said.
"The ambulances may be coming soon and we don't know where to put them in the hospital at this point.
"Because we're doubled and tripled up."
'We're secure and we're prepared for this'
Hotels along Interstate Highway 75, which runs up and down Florida's west coast, were jam packed with people seeking shelter.
The area is dotted with mobile home parks, which most residents had abandoned, taking refuge in local schools and other facilities being used as emergency shelters.
The area's numerous assisted-living facilities were mostly evacuated, too.
Heartis Venice, an assisted-living home north of Venice, was an exception.
Of its 107 residents, 98 decided to shelter in place and continue receiving care with the help of staff and some family members, general manager Michelle Barger said.
The facility, opened two years ago, was built to withstand a category five storm.
"Our community is locked down. We're secure and we're prepared for this," Ms Barger said, adding the facility was stocked up with enough food, water and medical supplies to last more than seven days.
"We feel pretty confident and safe, as do the residents and families and team members here."
Migrant boat sinks, leaving 23 missing
US Border Patrol said on Wednesday that 23 people were missing off the coast of Florida after a Cuban migrant boat sank due to the hurricane.
The US Coast Guard found three survivors about 3 kilometres south of the Florida Keys.
Four other Cubans swam to Stock Island, just east of Key West, the US Border Patrol said.
Local media said those four were hospitalised.
Aircrews continued to search for the remaining migrants.
Hurricane hunters fly through storm
As Hurricane Ian intensified on its way toward the Florida coast, hurricane hunters were in the sky doing something almost unimaginable: flying through the centre of the storm.
With each pass, the scientists aboard these planes take measurements that satellites cannot and send them to forecasters at the National Hurricane Center.
Video released by an engineer with National Oceanic and Atmospheric Administration (NOAA) and hurricane hunter Nick Underwood showed his crew aboard an aircraft nicknamed "Kermit" encountering a bout of severe turbulence as they flew through the eye of Hurricane Ian.
Kermit, the four-engine P-3 Orion turboprop aircraft, plunged into the eye of Ian hours before it ploughed into Florida's coast with catastrophic force.
Jason Dunion, a University of Miami meteorologist who leads the National Oceanic and Atmospheric Administration’s 2022 hurricane field program, described what hurricane hunters do.
"Basically, we’re take a flying laboratory into the heart of the hurricane, all the way up to category fives. While we’re flying, we’re crunching data and sending it to forecasters and climate modellers," he said.
"In the P-3s, we routinely cut through the middle of the storm, right into the eye."
ABC/wires
