Hurricane Katrina slammed into Florida's densely populated southeastern coast Thursday with sustained winds of 80 mph and driving rain. Four people were killed -- three by falling trees.

The storm strengthened into a Category 1 hurricane just before it made landfall along the Miami-Dade County-Broward County line, between Hallandale Beach and North Miami Beach. Weather officials said flooding was the main concern, as the storm dropped up to 15 inches in parts of Miami-Dade County.

Rain fell in horizontal sheets and blew gusts of up to 92 mph, toppling trees and street signs. Waves were estimated at 15 feet, and blowing sand covered waterfront streets. Florida Power & Light said more than 1.3 million customers were without electricity.

An overpass under construction in Miami-Dade County collapsed onto a highway, authorities said. No injuries were immediately reported.

Late Thursday, Katrina had weakened slightly to 75 mph and was about 20 miles northwest of Homestead in Miami-Dade County, heading southwest at 8 mph. About 5.9 million Florida residents were in the storm's projected path.

The storm killed three people who ignored official warnings to stay inside. A man in his twenties in Fort Lauderdale was crushed by a falling tree as he sat alone in his car. A 54-year-old man was killed by a falling tree in the Fort Lauderdale suburb of Plantation. A woman who was struck by a tree died at a hospital in Hollywood, hospital officials said.

A 79-year-old man in Cooper City died when his car struck a tree, Broward County officials said. No other details were immediately available.

"The message needs to be very clear: It's not a good night to be out driving around," National Hurricane Center Director Max Mayfield said. "The back side of the core of the hurricane has yet to come."

Three storm-related trauma victims were being treated at Memorial Regional Hospital in Hollywood, including a driver in critical condition after a tree fell on a car, a spokesman said.

The usually bustling streets of Miami Beach, a tourist haven, were largely deserted as the storm pounded the area. The city is hosting celebrities and partygoers in town for the MTV Video Music Awards. MTV called off its pre-awards festivities Thursday and Friday.

"It's like a ghost town out here," said Mark Darress, concierge at the Hotel Astor in Miami Beach. "I see the random, not-so-smart people riding scooters every now and then."

Many people were stranded at Miami and Fort Lauderdale airports, which closed Thursday night.

Katrina is the second hurricane in the state this year -- Dennis hit the Panhandle last month -- and the sixth since Aug. 13, 2004. Katrina formed Wednesday over the Bahamas and was expected to cross Florida before heading into the Gulf of Mexico.

Strong winds from Hurricane Katrina lash visitors at South Beach Park in Boca Raton. The storm knocked out electricity to more than 1.3 million Floridians.