Hurricane Hermine made landfall in Florida's Big Bend area early on Friday - the first hurricane to hit the state in more than a decade.
The Category 1 storm hit just east of St Marks at about 1.30am EDT(0530 GMT) with winds of around 80mph (129kph), according to the US National Hurricane Centre.
Projected storm surges of up to 12ft (3.7m) threatened a wide swathe of the coast and an expected drenching of up to 10in (25.4cm) of rain carried the danger of flooding along the storm's path over land, including the state capital, Tallahassee, which had not been hit by a hurricane since Kate in 1985.
As of 2am EDT (0600 GMT) on Friday, Hermine was centred about 35 miles (56km) south-east of Tallahassee, and was moving north-northeast. It was expected to drop back down to a tropical storm before pushing into Georgia, the Carolinas and up the East Coast with the potential for heavy rain and deadly flooding.
The last hurricane to strike Florida was Wilma, a powerful Category 3 storm which arrived on October 24 2005. It swept across the Everglades and struck heavily populated south Florida, causing five deaths in the state and an estimated 23 billion US dollars of damage.
Many took no chances with Hermine.
Tallahassee resident Tom Duffy, 70, said on Thursday that he planned to reserve a hotel room for Friday night in the neighbouring state of Alabama if downed trees caused the kind of power cuts he expected. Even before the storm's final approach to land, the city government tweeted that there were already some 32,000 outages reported around the capital city.
"We've dodged bullet after bullet after bullet," Mr Duffy said, but added that Hermine has taken "dead aim" at the city, where blustery winds sent trees swaying before dawn.
In Carrabelle, on the coast just 60 miles (97km) south-west of Tallahassee, Courtney Chason was keeping an eye on the storm surge as docks and boat houses were slowly being battered as the storm approached late on Thursday.
"I've never seen it this high, it's pretty damn crazy. I've been in this area for 30 years but I've never seen it like this. I hope it doesn't get any higher, we need lots of prayers."
Residents on some islands and other low-lying, flood-prone areas in Florida had been urged to clear out. Flooding was expected across a wide swathe of the marshy coastline of the Big Bend - the mostly rural and lightly populated corner where the Florida peninsula meets the Panhandle.
Florida Governor Rick Scott warned of the danger of strong storm surges, high winds, downed trees and power outages, and urged people to move to inland shelters if necessary and make sure they have enough food, water and medicine.
"You can rebuild a home, you can rebuild property, you cannot rebuild a life," he said at a news conference on Thursday afternoon, adding that "we are going to see a lot of flooding".
Mr Scott, who declared an emergency in 51 counties, said 6,000 National Guardsmen were poised to mobilise for the storm's aftermath. The governors of Georgia and North Carolina also declared states of emergency.
Across the Florida line in south Georgia, about a dozen people had already arrived by Thursday evening at a Red Cross shelter that opened at a city auditorium in Valdosta which is normally used for banquets and gospel concerts.
Cynthia Arnold left her mobile home for the shelter with her brother and her five-year-old grandson, saying: "I'm not just going to sit there and be ignorant."
Rainfall of 4in (10cm) to 10in (25.4cm) were possible along the coasts of Georgia and the Carolinas by Sunday. Lesser amounts were forecast farther up the Atlantic Coast, because the storm was expected to veer out to sea.