(Adds details of flooding in Florida, Georgia and South Carolina, Orlando flood rescues, U.S. military concerns about possibly stranded Keys residents, revised power outage figures)
* Irma downgraded to tropical storm, heads into Georgia
* Over 7.4 million in Florida, other states without power
* Nearly 40 lives claimed in Caribbean, at least 3 in U.S.
* Flooding hits coastal communities in three states
FLORIDA CITY/MARCO ISLAND, Fla., Sept 11 (Reuters) - S torm-shocked Floridians returned to shattered homes on Monday as the remnants of Hurricane Irma pushed inland, leaving more than half of all state residents without power and city streets underwater from Orlando and Jacksonville into coastal Georgia and South Carolina.
Downgraded to a tropical storm early on Monday, Irma had ranked as one of the most powerful Atlantic hurricanes on record before barreling into the Florida Keys on Sunday and plowing northward along the Gulf Coast to wreak havoc across a wide swath of the third-most populous U.S. state.
Especially hard hit was the resort archipelago of the Keys, extending into the Gulf of Mexico from the tip of Florida's peninsula and connected to the mainland by a single, narrow highway, Governor Rick Scott told a news conference on Monday.
"There's devastation," he said, adding that virtually every mobile-home park on the island chain was left upended. "It's horrible what we saw."
While some evacuees from the Keys expressed anger at authorities refusing to allow them to return to their homes, the U.S. Defense Department said as many as 10,000 residents who had stayed put on the island may now be stranded and in need of evacuation.
No timetable for reopening the Keys has been given.
In Miami, which escaped the worst of Irma's winds but experienced heavy flooding, residents in the city's Little Haiti neighborhood returned to the wreckage of trailer homes that were shredded by the storm.
"I wanted to cry, but this is what it is, this is life," Melida Hernandez, 67, who had ridden out the storm at a nearby church, said as she gazed at the ruins of her dwelling, split in two by a fallen tree.
Severe flooding was reported on Monday in northeastern Florida, including Jacksonville, where police were rescuing residents from waist-deep water as the St. Johns River rose to levels unseen since 1846.
"Stay inside. Go up. Not out," Jacksonville's website warned residents. "There is flooding throughout the city." The city also warned residents to be wary of snakes and alligators driven into the floodwaters.
Before dawn on Monday, Florida National Guard troops and local firefighters rescued nearly 150 people from dozens of flooded homes in western Orlando and ferried them in large trucks and rescue launches to a shelter on higher ground.
As Irma crossed into Georgia later in the day, packing sustained winds of 60 miles per hour (100 kph), the storm's tidal surge unleashed flooding in coastal communities from Brunswick and Savannah, Georgia, to Charleston, South Carolina. In Charleston, waves crashed over the city's Battery sea wall and filled downtown streets for blocks.
EVACUEES URGED TO STAY PUT
Still, the scope of damage in Florida and neighboring states paled in comparison with the utter devastation left by Irma as a rare Category 5 hurricane in parts of the Caribbean, where the storm killed nearly 40 people - at least 10 of them in Cuba - before turning its fury on Florida.
The storm claimed its first known U.S. fatality over the weekend in the Keys - a man found dead in a pickup truck that had crashed into a tree in high winds.
At least one other possibly storm-related fatal car crash was reported on Sunday in Orange County, Florida. On Monday, two people were killed by falling trees in two Atlanta suburbs, according to local authorities.
Some 6.5 million people, about one-third of Florida's population, had been ordered from their homes ahead of Irma's arrival, and more than 200,000 people sought refuge in about 700 shelters, according to state data.
Scott urged evacuees all over the state to stay put for now rather than rush home, saying downed power lines, debris and other hazards abounded. "Don't put any more lives at risk," he said.
One of the biggest lingering problems was widespread power outages, with utilities reporting more than 7.4 million homes and businesses without electricity in Florida and neighboring states. They said it could take weeks to fully restore service.
Scott said 65 percent of Florida was without power.
Travel into and out of the state likewise remained stymied. Miami International Airport, one of the busiest in the country, halted passenger flights through at least Monday.
Police in Miami-Dade County and Fort Lauderdale reported making 48 arrests for looting.
The Keys were largely evacuated before the storm struck with winds of up to 130 mph (209 kph), and police established a checkpoint on Monday to keep displaced residents from returning while authorities work to restore power, water, fuel supplies and medical service.
Some seeking re-entry argued with police who stopped them at the first of a series of bridges leading to the island chain.
"I would expect that the Keys are not fit for re-entry for regular citizenry for weeks," Tom Bossert, a homeland security adviser to U.S. President Donald Trump, told a White House news briefing.
Irma's arrival in Florida came about two weeks after Hurricane Harvey claimed about 60 lives and caused property damage estimates as high as $180 billion after pummeling the Gulf Coasts of Texas and Louisiana with heavy rains and severe flooding.
Insured property losses in Florida from Irma are expected to run from $20 billion to $40 billion, catastrophe modeling firm AIR Worldwide estimated.
That tally, lower than earlier forecasts of up to $50 billion in insured losses, helped spur a relief rally on Wall Street as fears eased that Irma would cut into U.S. economic growth.
As shelters began to empty on Monday, some 7,000 people filed out of Germain Arena in Estero, south of Fort Myers. The crowd included Don Sciarretta, 73, and his 90-year-old friend, Elsie Johnston, who suffers from Alzheimers disease.
"For the next storm, I'll go somewhere on my own like a hotel or a friend's house," Sciarretta said. "I'm not going through this again."
(Additional reporting by Daniel Trotta in Orlando, Fla., Bernie Woodall, Ben Gruber and Zachary Fagenson in Miami, Dan Whitcomb in Los Angeles, Letitia Stein in Detroit, Colleen Jenkins in Winston-Salem, N.C., Harriet McLeod in Mt. Pleasant, S.C., Doina Chiacu and Jeff Mason in Washington, Scott DiSavino in New York and Marc Frank in Havana; Writing by Scott Malone and Steve Gorman; Editing by Paul Simao and Peter Cooney)