Updated 12.30pm

Hurricane Irma pounded heavily populated areas of central Florida today 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 yesterday and battered towns as it worked its way up the state.

The storm gradually lost strength, weakening to a Category 1 hurricane by 2am ET (0600 GMT) today, the National Hurricane Center said. By 5am ET (0900 GMT), Irma was churning northwest in the centre of the state and was about 60 miles (100 km) north of Tampa, with maximum sustained winds of near 75 miles per hour (120 km per hour.)

A large area 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.

WATCH: Hurricane Irma pounds Cuba

Florida Director of Emergency Management Bryan Koon said officials would wait until first light on Monday to begin rescue efforts and assess damage, adding he did not have yet any numbers on fatalities statewide, the Miami Herald reported.

Damage appeared to be severe in the Florida Keys, where Irma first came ashore in the state 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.

Senterfitt told a teleconference a large airborne relief operation was being prepared by the Air Force and Air National Guard 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 picturesque town of Key West.

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 its Twitter feed.

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On Sunday, Irma claimed its first US 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, devastating several small islands, and grazing Puerto Rico, the Dominican Republic and Haiti before pummeling parts of Cuba's north coast with 36-foot-tall (11-meter) waves.

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. Its ferocity as it bore down on hurricane-prone Florida prompted one of the largest evacuations in US history.

Some 6.5 million people, about a third of the state's population, had been ordered to evacuate southern Florida to shelters, hotels or relatives in safer areas.

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 we’re OK, knock on wood."

Millions without power

High winds snapped power lines and left about 5.8 million Florida homes and businesses without power, data from the state 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. The airport said in a Twitter post that after assessing damage, it would determine if flights could resume on Tuesday.

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 cms) of rain, it said. 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.

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.

Miami apartment towers swayed in the high winds on Sunday, three construction cranes were toppled, and small white-capped waves could be seen in flooded streets between Miami office towers belted by powerful winds.

"We feel the building swaying all the time," restaurant owner Deme Lomas said in a phone interview from his 35th-floor apartment. "It's like being on a ship."

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 that is a major tourist destination and that accounts for about 5 percent of US gross domestic product. Thousands of flights were cancelled.

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 US 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.