The hurricane eye, currently with maximum-sustained winds of 125 mph, churned toward southeastern Alabama and southwestern Georgia, the National Hurricane Center said.
Storm surge to dangerous level, neighborhoods submerged and some buildings crumbled under the gusts, images and videos online showed.
As of Wednesday afternoon, nearly 200,000 customers lost power in Florida, local media reported.
The hurricane is the most powerful storm on record to hit Florida's Panhandle, a region rimmed with tourist beaches and fishing villages along the Gulf of Mexico.
"This is an extremely dangerous and life-threatening situation," the National Weather Service has said, noting Michael grew from a tropical storm to a Category 4 hurricane in about 40 hours.
More than 6 million people in Florida were forced to evacuate last year ahead of the landfall of Hurricane Irma, which was ranked one of the costliest storms in U.S. history with repair bills reportedly up to more than 60 billion dollars.