* 5.8 million without power across Florida
* Hurricane weakens as it heads northwest
* Coastal areas vulnerable to storm surge
* Miami airport closed on Monday
TAMPA, Fla.,/MIAMI, Sept 11 (Reuters) - Hurricane Irma pounded heavily populated areas of central Florida on Monday as it carved through the state with high winds, storm surges and torrential rains that left millions without power, ripped roofs off homes and flooded city streets.
Irma, once ranked as one of the most powerful hurricanes recorded in the Atlantic, came ashore in Florida on Sunday and battered towns as it worked its way up the state.
The storm gradually lost strength, weakening to a Category 1 hurricane overnight, the National Hurricane Center said. By 5 a.m. ET (0900 GMT), Irma was churning northwest in the center of the state and was about 60 miles (100 km) north of Tampa, with maximum sustained winds of 75 miles per hour (120 km per hour.)
Much of the state's east and west coasts remained vulnerable to storm surges, when hurricanes push ocean water dangerously over normal levels. That risk extended to the coast of Georgia and parts of South Carolina, the hurricane center said.
Officials planned to wait until first light on Monday to begin rescue efforts and assess damage, the Miami Herald cited Florida Director of Emergency Management Bryan Koon as saying. He did not give any numbers on possible fatalities.
Damage appeared to be severe in the Florida Keys, where Irma first came ashore as a Category 4 hurricane with sustained winds of up to 130 mph (215 kph) in the early hours of Sunday, the paper quoted Monroe County Emergency Director Martin Senterfitt as saying.
A large military airborne relief operation was being prepared to take help to the chain of islands, which are linked by a dramatic series of bridges and causeways from Key Largo almost 100 miles (160 km) southwest to the city of Key West, Senterfitt told a teleconference.
Early on Monday, Irma brought gusts of up to 100 mph (160 kph) and torrential rain to areas around Orlando, one of the most popular areas for tourism in Florida because of its cluster of theme parks, the National Weather Service said.
In Daytona Beach, a city on the east coast about 55 miles (90 km) northeast of Orlando, city streets were flooded and emergency authorities carried out several water rescues, the Daytona Beach Police Department said on Twitter.
Over the weekend, Irma claimed its first U.S. fatality - a man found dead in a pickup truck that had crashed into a tree in high winds in the town of Marathon, in the Florida Keys, local officials said.
The storm killed at least 28 people as it raged westward through the Caribbean en route to Florida. Irma was ranked a Category 5, the rare top end of the scale of hurricane intensity, for days, and carried maximum sustained winds of up to 185 mph (295 kph) when it crashed into the island of Barbuda on Wednesday.
Ahead of Irma's arrival, some 6.5 million people in southern Florida, about a third of the state's population, were ordered to evacuate their homes.
Jonathan Brubaker, 51, waited out the storm bunkered in a recently constructed house in Bradenton, on the state's west coast, with hurricane shutters drawn, flashlights and candles ready. As a radar app on his phone showed Irma passing by, he had seen little more than gusty winds. He still had power.
"I feel like we kind of dodged bullet on this one," he said, adding that he would wait until Monday morning before trying to sleep. "And then, I think were OK, knock on wood."
MILLIONS WITHOUT POWER
High winds snapped power lines and left about 5.8 million Florida homes and businesses without power, state data showed.
Many of the evacuation orders extended until at least Monday due in part to flooding, massive power outages and downed electric lines, leaving residents unable to return to their homes to survey any damage.
TV news video of damage in Naples, a city on the Gulf coast about 125 miles (200 km) northwest of Miami, showed buildings ripped apart by winds and streets flooded by rain and storm surges.
Miami International Airport, one of the busiest in the country, halted passenger flights through at least Monday.
Irma was forecast to weaken to a tropical storm as it moved near Florida's northwestern coast on Monday morning, the National Hurricane Center said. It would cross the eastern Florida Panhandle and move into southern Georgia later in the day, dumping as much as 16 inches (41 cm) of rain, it said.
SKYSCRAPERS SWAY IN LASHING WINDS
Five tornados were reported in Florida on Sunday, causing damage to several structures but there were no indications of anyone being seriously injured, the National Weather Service said.
Along with hurricane warnings and watches in Florida, the weather service placed tropical storm warnings for large parts of Alabama, Georgia and South Carolina.
The densely populated Miami area was spared the brunt of Irma, although the hurricane's wide reach meant the state's biggest city was still battered.
Miami apartment towers swayed in the high winds on Sunday, three construction cranes were toppled, and streets flooded between office towers.
Police in Miami-Dade County said they had made 29 arrests for looting and burglary.
The storm and evacuation orders caused major disruption to transportation in the state, which is a major tourist destination and which accounts for about 5 percent of U.S. gross domestic product.
Irma, which hit just days after the Houston area was deluged by unprecedented flooding in Texas from Hurricane Harvey, was expected to cause billions of dollars in damage to the third-most-populous U.S. state.
However, European shares rose on Monday in early deals, led higher by insurers as the weakening of Irma raised expectations that costs for the industry might be lower than initially feared.
(Additional reporting by Bernie Woodall, Ben Gruber and Andy Sullivan in Miami, Letitia Stein in Detroit, Colleen Jenkins in Winston-Salem, N.C., David Shepardson, Doina Chiacu and Roberta Rampton in Washington and Scott DiSavino in New York; Writing by Scott Malone and Jon Herskovitz; Editing by Frances Kerry)