Leaving at least 478 people dead, mostly in Haiti, in its wake across the Caribbean, Hurricane Matthew steamed toward Florida with potentially catastrophic winds of 140 mph (220 kph) on Thursday as US President Barack Obama declared a state of emergency in the state ahead of the hurricane.
Around 3,862 flights have been scheduled to be cancelled between Wednesday and Saturday.
It was the most powerful storm to threaten the U.S. Atlantic coast in more than a decade.
"This storm's a monster," Gov. Rick Scott warned as it started lashing the state with rain and wind around nightfall. He added: "I'm going to pray for everybody's safety."
As it moved north in the evening, Matthew stayed about 100 miles or more off South Florida, sparing the 4.4 million people in the Miami and Fort Lauderdale areas from its most punishing effects.
"We were lucky this time," Miami-Dade Mayor Carlos Gimenez said.
The hurricane was instead expected to blow ashore — or come dangerously close to doing so — early Friday north of West Palm Beach, which has about 1.1 million people, and then slowly push north for the next 12 hours along the Interstate 95 corridor, through Cape Canaveral and Jacksonville, according to the National Hurricane Center.
Forecasters said it would then probably hug the coast of Georgia and South Carolina over the weekend before veering out to sea — perhaps even looping back toward Florida in the middle of next week as a tropical storm.
Millions of people in Florida, Georgia and South Carolina were told to evacuate their homes, and interstate highways were turned into one-way routes to speed the exodus. Florida alone accounted for about 1.5 million of those told to clear out.
Roads in Florida, Georgia and North and South Carolina were jammed and gas stations and food stores ran out of supplies as the storm approached with not just high winds but strong storm surges and drenching rain.
The hurricane carried winds of 220 kph which made it a Category 4 hurricane and is likely to remain so as it approached the United States, where it could either take direct aim at Florida or brush along the state's coast through Friday night.
"The storm has already killed people. We should expect the same impact in Florida," the governor warned.
Many boarded up their homes and businesses and left them to the mercy of the storm.
Due to the emergency declaration in Florida, the Department of Homeland Security and Federal Emergency Management Agency are authorised to coordinate all disaster relief efforts, Xinhua news agency reported.
Meanwhile, local authorities issued a mandatory evacuation order on Thursday for nearly 1.5 million coastal residents in preparation for the hurricane, one of the most powerful storms to hit the region in a decade.
(With inputs from agencies)