Hurricane Harvey was downgraded to a tropical storm on Saturday as it slowly moved inland after hitting the US state of Texas with torrential rain and heavy winds.

At least one person has died as a result of the storm, according to the mayor of Rockport, Texas.

The storm is expected to cause massive flooding after battering the coast with 209km-per-hour winds.

Harvey is the strongest storm to hit the US state in more than 50 years.

It is expected to linger for days over Texas and bring more than 100cm of rain to some parts of the southern state.

The mayor of Houston said the city could receive as much as five to 7.5cm of rain an hour into Sunday night.

Parts of the state also experienced tornados spawned by Harvey, causing damage to homes, vehicles and buildings.

The latest forecast storm track has Harvey looping back towards the Gulf of Mexico coast before meandering north again on Tuesday.

Nearly 25cm of rain had already fallen in a few areas in southeastern Texas, the centre said. Flash floods have already hit some areas, the National Weather Service said.

As many as six million people were believed to be in Harvey's path, as is the heart of the United States' oil-refining operations.

The seaside town of Rockport, 48km north of the city of Corpus Christi, was hit hard.

Several homes had collapsed and many more buildings suffered damage. Roofs had been ripped off some and windows blown in.

Streets were flooded and strewn with power lines and debris. At a recreational vehicle sales lot, a dozen automobile homes were flipped over and one had been blown into the middle of the road.

While thousands fled the expected devastating flooding and destruction, many residents stayed put in imperilled towns and stocked up on food, fuel and sandbags.

"It was terrible," resident Joel Valdez, 57, told Reuters news agency. The storm ripped part of the roof from his trailer home. I could feel the whole house move."

Valdez said he stayed through the storm to look after his animals.

"I have these miniature donkeys and I don't know where they are," he said, sitting in a Jeep with windows smashed by the storm.

Resident Frank Cook, 56, also stayed through the hurricane.

"If you have something left of your house, you're lucky," he said, surveying the damage from his vehicle.

Before the storm hit, Rockport's mayor told anyone staying behind to write their names on their arms for identification purposes - in case of death or serious injury.

The coastal city of Port Lavaca, farther north on the coast, had no power and some streets were flooded.

"There is so much tree damage and debris that the cost of cleanup will be enormous," Mayor Jack Whitlow said after touring the city earlier Saturday.

The hurricane came ashore near Port Lavaca late on Friday with maximum winds of 209 km/h.

That made it a Category 4 storm on the Saffir-Simpson scale, the second-highest category and the most powerful storm in more than a decade to hit the mainland United States.

The streets of Corpus Christi, which has about 320,000 residents, were deserted with billboards twisted and strong winds still blowing.

City authorities asked residents to reduce their use of toilets and faucets because power outages left waste water plants unable to treat sewage.

The city also asked residents to boil water before consumption.