* Category 4 hurricane hits U.S. Texas state coast Friday night
* Storm weakens early Saturday as moves inland
* Utilities report quarter of a million customers without power
* Flash floods reported, authorities warn of catastrophic flooding (Updates throughout)
CORPUS CHRISTI, Texas, Aug 26 (Reuters) - The most powerful hurricane to hit the U.S. state of Texas in more than 50 years moved slowly inland on Saturday, dumping torrential rain expected to cause catastrophic flooding after battering the coast with 130 miles per hour winds.
As dawn breaks in the southern United States, a clearer picture will emerge of the damage wrought by Hurricane Harvey. Texas utility companies said nearly a quarter of a million customers were without power.
Harvey is the strongest storm to hit Texas, the center of the U.S. oil and gas industry, since 1961, and residents and emergency respondents were still sheltering early Saturday.
The town of Rockport, 30 miles (48 km) north of the city of Corpus Christ, appeared to be one of the hardest hit. Ahead of the storm's arrival, the city's mayor told anyone staying behind to write their names on their arms for identification purposes in case of death or injury.
"Right now we're still hunkered down and can't go anywhere," said Steve Sims, the volunteer fire chief in Rockport. "We've heard rumors of 1,000 different things, we can't confirm anything because we haven't seen anything. We know we've got a lot of problems, but we don't know what yet."
A high school, hotel, senior housing complex and other buildings suffered structural damage, according to emergency officials and local media. Some were being used as shelters.
Sims said power, internet and most cell phone service was out in the town of 10,000 where about two-thirds of residents evacuated. Most of the senior citizens and nursing homes were among the first to be evacuated, he said.
The hurricane came ashore northeast of Corpus Christi late on Friday with maximum winds of 130 miles per hour (209 km per hour). That made it a Category 4 storm on the Saffir-Simpson scale, the most powerful storm in over a decade to hit the mainland United States.
HEADING INLAND, STORM WEAKENS
The storm weakened early Saturday, but was still a strong hurricane as it moved inland at about six mph (10 km per hour), the U.S. National Hurricane Center said.
Harvey was expected to blow across the coast and up through Louisiana for days, with forecasts for storm surges of up to 13 feet (four meters) and over three feet (90 cm) of rain.
Nearly 10 inches of rain had already fallen in a few areas in southeastern Texas, the center said. Some areas have already seen flash floods, the National Weather Service said.
As many as 6 million people were believed to be in Harvey's path, as is the heart of America's oil-refining operations. The storm's impact on refineries has already pushed up gasoline prices. The U.S. Environmental Protection Agency eased rules on gasoline specifications late Friday to reduce shortages.
Donald Trump, facing the first big natural disaster of his presidency, said on Twitter he signed a disaster proclamation which "unleashes the full force of government help" shortly before Harvey made landfall.
"In the dark, internet out, ham radio not working. Is anybody out there? Alone trying not to be scared," Donna McClure in Corpus Christi said on Twitter as the storm made landfall.
Utilities American Electric Power and CenterPoint Energy reported a combined total of more than 240,000 customers without power.
While thousands fled the expected devastating flooding and destruction, many residents stayed put in imperiled towns and stocked up on food, fuel and sandbags.
HOUSTON PREPARES FOR FLOODS
As a Category 4 hurricane, Harvey is the first major hurricane of Category 3 or more to hit the mainland United States since Hurricane Wilma struck Florida in 2005.
Its size and strength also dredged up memories of Katrina, the 2005 hurricane that made a direct hit on New Orleans as a Category 3 storm, causing levees and flood walls to fail in dozens of places. About 1,800 died in the disaster made worse by a slow government emergency response.
Corpus Christi, a city of 320,000, was under voluntary evacuation for Harvey.
Texas and Louisiana declared states of disaster before the storm hit, authorizing the use of state resources to prepare.
The NHC's latest tracking model shows the storm sitting southwest of Houston for days, giving the nation's fourth most populous city a long dousing of rain and wind. Residents of Houston were woken with automatic flash flood warnings to their cell phones early Saturday.
The city warned residents of flooding from close to 20 inches (60 cm) of rain over several days.
GASOLINE PRICES SPIKE
Gasoline stations on the south Texas coast were running out of fuel as residents fled the region. U.S. gasoline prices spiked as the storm shut down several refineries and 22 percent of Gulf of Mexico oil production, according to the U.S. government.
More than 45 percent of the country's refining capacity is along the U.S. Gulf Coast, and nearly a fifth of the nation's crude oil is produced offshore.
Ports from Corpus Christi to Texas City, Texas, were closed to incoming vessels and Royal Dutch Shell Plc, Anadarko Petroleum Corp, Exxon Mobil Corp and others have evacuated staff from offshore oil and gas platforms.
Concern that Harvey could cause shortages in fuel supply drove benchmark gasoline prices to their highest in four months. Profit margins for making gasoline <RBc1-CLc1> hit their strongest levels in five years for this time of year.
The U.S. government said it would make emergency stockpiles of crude available if needed to plug disruptions. It has regularly used them to dampen the impact of previous storms on energy supplies.
(Reporting by Brian Thevenot; Additional reporting by Jessica Resnick-Ault in New York and Liz Hampton in Texas; Writing by Brendan O'Brien and Simon Webb; Editing by Mary Milliken and Helen Popper)