Hurricane Idalia made landfall in Florida as an "extremely dangerous" Category 3 storm on Wednesday after millions of residents evacuated or hunkered down in homes and shelters to await the arrival of a life-threatening storm surge.

Drawing strength from the Gulf of Mexico's warm waters, Idalia was forecast to unleash destructive winds and torrential downpours that will cause coastal flooding up to 16 feet (4.88 meters) deep.

"Don't put your life at risk by doing anything dumb at this point. This thing's powerful," Governor Ron DeSantis said during a morning news briefing in Tallahassee that was interrupted for a few seconds by a power cut.

The National Hurricane Center (NHC) said Idalia made landfall at 7:45 a.m. (0945 GMT) at Keaton Beach in Florida's Big Bend region, where the state's northern Gulf Coast panhandle curves into the western side of the Florida Peninsula, roughly bounded by the inland cities of Gainesville and Tallahassee, the state capital.

Overnight, Idalia attained "an extremely dangerous Category 4 intensity" on the five-step Saffir-Simpson wind scale on its way to landfall in Florida Wednesday morning, the NHC in Miami said.

But as of 7 a.m. (0900 GMT) it weakened slightly, slipping into Category 3, with maximum sustained winds of 125 mph (201 kmph). Any storm reaching Category 3 or higher is considered a major hurricane.

Most of Florida's 21 million residents, and many in the adjacent states of Georgia and South Carolina, were under hurricane warnings and other storm-related advisories. State emergency declarations were issued in all three.

As the hurricane pummeled the state, over a quarter of a million customers are without power in Florida, according to the latest update on the website PowerOutage.us.

(With input from Reuters)