Packing 145-kph winds, the storm on Saturday promised to dump torrential rain on the peninsula for up to 15 hours in what emergency officials said could be a "marathon" of anxiety and devastation.
Blinding rain squalls drove clouds of sand into the air while huge waves thundered onto the beaches of Florida's Atlantic coast, where 2.5 million people had been told to evacuate.
"We don't want to have people lose their lives because of abject stupidity," said Florida Governor Jeb Bush.
There are reports indicating 2 million Floridians have already been cut off from electric power.
Before hitting Florida, Frances killed two people in the Bahamas, blowing off roofs, downing trees and power lines and causing widespread flooding in the 700-island archipelago.
Frances was massive and could bring flooding, unlike Hurricane Charley which ripped through Florida three weeks ago.
Florida Power and Light spokesman Bill Swank said 459,000 customers were without power along the Florida east coast.
The hurricane may cause between $2 billion and $10 billion in insured losses, according to industry group Risk Management Solutions.
Few stores remained open in the ghost towns along the coast. Most airports across the state were closed. Disney World and other central Florida theme parks were closed and big shopping malls were shut as well.