(Adds comments from people in storm's path)
* Florida orders evacuation of 6.3 million residents
* Coastal flooding hits northern Cuba
* Storm due to head up Florida's west coast
* Downgraded to Category 3 but expected to strengthen
FORT MYERS, Fla., Sept 9 (Reuters) - Hurricane Irma turned its menacing sight toward the Florida Keys on Saturday as it completed a destructive march along Cuba's northern coast and set off an 11th hour scramble for safety by Americans who may have ignored warnings to evacuate.
Irma, one of the fiercest Atlantic storms in a century, was expected to rip through Florida's southern archipelago on Sunday morning. It will make landfall on the Florida peninsula somewhere west of Miami and then head up the state's west coast, by Tampa, forecasters said.
Irma, which has killed at least 22 people in the Caribbean, was likely to inflict billions of dollars in damage in the third largest U.S. state by population.
On Florida's west coast, Charley Ball said he expected a storm surge to engulf the entire island of Sanibel, an affluent community where he lives and operates a painting company.
"Just left the island and said goodbye to everything I own," the 62-year-old Ball said.
The storm could bring winds in excess of 130 mph (209 kph) and a storm surge up to 15 feet (4.6 meters), which is expected to trigger flooding.
Irma, located about 115 miles (185 km) southeast of Key West on Saturday afternoon, was a Category 5 storm, the highest ranking possible, when it crashed into Cuba during the morning. It gradually weakened to a Category 3 storm as it bumped along the island's northern coastline, flooding streets and sending waves crashing over sea walls.
Maximum sustained winds stayed at around 125 mph (201 kph), the U.S. National Hurricane Center (NHC) said.
Irma is expected to regain strength as it steams over warm waters south of Florida, according to the NHC. It was likely to regain Category 4 status before striking Florida, said NHC spokesman Dennis Feltgen.
The wide reach of Irma's deadly winds and its size, more than its numerical ranking, has rattled veteran hurricane watchers. It could rival any storm in Florida's history, Governor Rick Scott said.
Irma will dump 10 inches to 20 inches (25 cm to 51 cm) of rain over Florida and southeast Georgia from Saturday through Monday, the National Weather Service said, a fraction of what Hurricane Harvey dropped on Texas and Louisiana last month.
But unlike with Harvey, dangerous winds will barely abate once Irma makes landfall on Sunday morning.
A long line of people in Estero, Florida lined up to enter an arena that officials converted into an evacuation shelter, one of hundreds that have opened up across the state.
Andrea Prather, a singer from the Gulf Coast city of Fort Myers, said she was confident that she would stay safe on the 25th floor of her high-rise building.
She closely followed reports about the storm's path as it headed toward her town.
"When it made that turn, that's when I had that anxiety pit in my stomach. I was like, 'OK this is not good,"' she said.
In Cuba, the destruction along the north central coast was similar to that seen on other Caribbean islands over the last week as Irma plowed into Ciego de Avila province.
In the storm's aftermath, people walked through ankle-deep water in Caibarien, a Cuban fishing town where streets were flooded and covered in seaweed. Elsewhere, winds toppled trees and utility polls or ripped apart roofs.
"It was powerful, water rose to about one meter high," said Risle Echemendia, 28, in Caibarien as he swept muddy water out of his front door. "This was the strongest storm Caibarien ever had. It will take a while to recover from this, at least a few years."
It was the first time the eye of a Category 5 storm had made landfall in Cuba since 1932, state media said, and the island's Communist government ordered the evacuation of more than 1 million people, with most sheltering with family and friends.
THIRD OF FLORIDA
Officials in Florida have ordered a total of 6.3 million people, or about a third of the state's population, to evacuate, creating massive traffic jams on highways and bringing huge crowds to shelters.
In Palm Beach, President Donald Trump's waterfront Mar-a-Lago estate was under evacuation order. Trump and his cabinet were at Camp David in Maryland, receiving updates on the storm.
Irma was already whipping south Florida with winds on Saturday, and the governor said thousands of people were without power.
About 9 million people in Florida may lose power, some for weeks, said Florida Power & Light Co, which serves almost half of the state's 20.6 million residents.
The window for people in evacuation zones to flee was drawing to a close on Saturday, officials said, warning that gas stations would soon be without fuel and bridges would be closed in some areas.
"You can't play chicken with this thing or try to outrun the storm," U.S. Senator Marco Rubio of Florida said at a news conference.
Rubio acknowledged some people had fled their homes only to arrive in areas expected to be in the storm's path, but he advised people in that situation to hunker down.
That was the case for Chris Cardona, 54, and his wife Laurie, who left their mobile home near Miami on Thursday to seek refuge with friends near Tampa.
"Not only did we go west, but so did Irma. She's tracking us, that feisty minx," Cardona said by phone.
Hurricane Irma could cause insurance losses of between $15 billion and $50 billion in the United States, catastrophe modeling firm AIR Worldwide said.
Irma was set to hit the United States two weeks after Hurricane Harvey struck Texas as a Category 4 storm, killing about 60 people and causing property damage estimated at up to $180 billion in Texas and Louisiana.
(Additional reporting by Sarah Marsh in Remedios, Marc Frank in Havana, Makini Brice in Cap-Haitien, Haiti,; Delana Isles in Providenciales, Turks and Caicos, Bernie Woodall, Ben Gruber and Andy Sullivan in Miami, Bate Felix, Richard Lough and Dominique Vidalon in Paris, Toby Sterling in Amsterdam and Neil Hartnell in Nassau, Bahamas; Writing by Alex Dobuzinskis and Frank McGurty; Editing by Diane Craft and Alistair Bell)