(Adds details on Florida Keys, updates storm position and strength)
* More than 6 million in Florida, other states without power
* Ten deaths reported in Cuba
* Irma downgraded to tropical storm as it heads northwest
* Fresh flooding in northeastern Florida cities
Miami airport closed on Monday
By Zachary Fagenson and Andy Sullivan
MIAMI/Florida City, Fla., Sept 11 (Reuters) - Downgraded from a hurricane to a tropical storm, Irma flooded several northern Florida cities with heavy rain and a high surge of seawater on Monday, as residents of the part of the state first walloped by the storm tried to return to their homes.
Irma, once ranked as one of the most powerful hurricanes recorded in the Atlantic, cut power to millions of people and ripped roofs off homes as it hit a wide swath of Florida over the past day.
With sustained winds of up to 65 miles per hour (105 kph), Irma was located about 70 miles (113 km) east of the Florida state capital Tallahassee and headed toward the Georgia border, the National Hurricane Center said at 11 a.m. ET (1500 GMT).
About two dozen vehicles filled with people who had fled the Florida Keys, where Irma roared ashore on Sunday with sustained winds of up to 130 mph (209 kph), lined up near the entrance to the highway that connects the archipelago to the mainland with a series of bridges and causeways.
They expressed anger at police who asked them to drive to a racetrack a few miles away to register before returning to their homes.
"This is how people are going to die - nobody's going to want to leave the Keys," shouted Shelby Bentley. "I've been in the Keys for 40 years ... It's the first time I've evacuated from a hurricane. It'll be my last time."
Officials in Monroe County, where the Keys are located, said that most of the islands still lacked fuel, electricity, running water and cell service on Monday.
"For many people, supplies are running low and anxiety is running high," the county said in a statement posted online. Inspection teams were working to clear bridges and ensure their safety. "Once the roads are cleared, and the bridges are inspected for use, aid and relief can start to move."
Irma hit Florida after powering through the Caribbean as a rare Category 5 hurricane, the top rung of the Saffir-Simpson scale. It killed 38 people, including 10 in Cuba, which was battered over the weekend by ferocious winds and 36-foot (11-meter) waves.
Northeastern Florida cities including Jacksonville were flooding on Monday, with city sheriffs pulling residents from waist-deep water.
"Stay inside. Go up. Not out," Jacksonville's website warned residents. "There is flooding throughout the city and more rain is expected."
After what she called a terrifying night bunkered in her house in St. Petersburg, on Florida's Gulf Coast, with her children and extended family, Julie Hally emerged with relief on Monday. The winds had toppled some large tree branches and part of a fence, but her house was undamaged.
"My heart just pounded out of my chest the whole time," said Hally, 37. "You hear stuff hitting your roof. It honestly sounds like somebody is just whistling at your window the whole night. It's really scary."
U.S. President Donald Trump, attending a ceremony at the Pentagon to remember the victims of the Sept. 11, 2001, attacks, vowed a full response to Irma as well as continued federal support for victims of Hurricane Harvey, which flooded Texas.
"These are storms of catastrophic severity and we are marshalling the full resources of the federal government to help our fellow Americans," Trump said. "When Americans are in need, Americans pull together and we are one country."
Florida's largest city, Miami, was spared the brunt of the storm but still suffered heavy flooding. Utility crews were on the streets there clearing downed trees and utility lines. All causeways leading to Miami Beach were closed by police.
Traveling through central Florida early on Monday, Irma brought gusts of up to 100 mph (161 kph) and torrential rain to areas around Orlando, one of the most popular areas for tourism in the state because of its cluster of theme parks, the National Weather Service said.
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.
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. About 200,000 were being housed in shelters, according to federal officials.
The storm did some $20 billion to $40 billion in damage to insured property as it tore through Florida, catastrophe modeling firm AIR Worldwide estimated.
That estimate, lower than earlier forecasts of up to $50 billion in insured losses, helped spur a relief rally on Wall Street as fears that Irma would cut into U.S. economic growth eased.
Shares of insurance companies were among the big winners, with Florida-based Federated National, HCI Group and Universal Insurance all up more than 13 percent.
High winds snapped power lines and left more than 6 million homes and businesses without power in Florida, Georgia, South Carolina and Alabama, according to state officials and utilities on Monday.
Miami International Airport, one of the busiest in the country, halted passenger flights through at least Monday. According to the FlightAware.com tracking site, a total of 3,582 U.S. flights were canceled on Monday, mostly as a result of the storm.
Police in Miami-Dade County said they had made 29 arrests for looting and burglary. Fort Lauderdale police said they had arrested 19 people for looting during the storm.
(Additional reporting by Bernie Woodall, Ben Gruber and Andy Sullivan in Miami, Letitia Stein in Detroit, Colleen Jenkins in Winston-Salem, N.C., Doina Chiacu and Jeff Mason in Washington, Scott DiSavino in New York and Marc Frank in Havana; Writing by Scott Malone and Jon Herskovitz; Editing by Frances Kerry and Paul Simao)