Hurricane Dennis made landfall around 2 p.m. Central Time near Pensacola, Fla. as a strong Category Three hurricane.
Max Mayfield, director of the hurricane center, said Dennis is unusually strong for a hurricane this early in the season.
"This is a very, very dangerous hurricane," Mayfield said. "No matter where this hurricane makes landfall, it's going to have a big, big impact over a very large area."
Dangerous storm surges will cover barrier islands and push into many bays from Alabama to Florida, Mayfield said.
Dennis is a bit stronger than Hurricane Ivan -- a Category 3 storm which pounded the Gulf Coast after it came ashore last September near Gulf Shores, Alabama.
Some 10 million Americans are potentially in Dennis' path, federal officials said.
A hurricane warning is in effect from Florida's Ochlockonee River west to the mouth of the Pearl River at the Mississippi-Louisiana border.