Hurricane Ike in Gulf of Mexico, headed for Texas

GALVESTON, Texas (Reuters) - Hundreds of thousands of people fled coastal areas in the path of Hurricane Ike on Thursday as the storm gathered strength on a collision course with the Texas Gulf Coast, threatening to swamp low-lying areas around Houston under a massive swell of water.

Ike was a Category 2 storm with 100 mph (160 kph) winds and likely will come ashore late on Friday or early on Saturday as a dangerous Category 3 storm on the five-step intensity scale with winds of more than 111 mph (178 kph), the National Hurricane Center said.

But because of its wide scope -- Ike is larger geographically than Hurricane Katrina was in 2005 -- it could bring a storm surge up to 20 feet, normally associated with larger storms.

"The most important message I can send is, do not take this storm lightly," U.S. Homeland Security Secretary Michael Chertoff told reporters in Washington. "This is not a storm to gamble with."

With the storm's track taking it away from the bulk of 4,000 offshore platforms that produce about a quarter of U.S. oil supply, U.S. crude oil futures dipped as low as $100.10 a barrel, the lowest level since early April.

The hurricane's current track would see it hit the Texas coast near Freeport in Brazoria County, just south of Galveston. It could be the worst storm to hit the Texas coast since Hurricane Carla came ashore near Corpus Christi in 1961.

The coastal areas under threat from Ike are lined with oil refineries and other heavy industrial facilities and some resort areas have million-dollar beachfront homes.

600,000 EVACUATED

State estimates show that up to 600,000 residents in low-lying coastal areas like the island of Galveston and Port Arthur left in the wake of mandatory evacuation orders.

By comparison, about 2 million people fled Louisiana coastal cities in the path of Hurricane Gustav, which hit on September 1.

The 2 million-plus residents of Houston, about 50 miles north of Galveston, could see hurricane-force winds as Ike moves inland but officials have ordered no widespread evacuations.

While New Orleans is below sea level, downtown Houston is about 50 feet above sea level but Houston's flat, expansive terrain still leaves it vulnerable to flooding.

"We're not talking about gently rising water. We are talking about a storm surge," Harris County Judge Ed Emmett said, ordering the evacuation of Galveston.

At 5 p.m. EDT on Thursday, the hurricane center said in its latest advisory Ike was 510 miles east-southeast of Corpus Christi, Texas, and about 400 miles east-southeast of Galveston. It was moving west-northwest at 10 mph (17 kph).

New Orleans, still scarred by Hurricane Katrina, which killed 1,500 people and caused $80 billion in damage on the U.S. Gulf Coast in 2005, appeared to be out of danger.

However, the center early on Thursday extended a tropical storm warning as far east as the Mississippi-Alabama border, including New Orleans. A hurricane watch remained in effect from south of Baffin Bay west to Port Mansfield, Texas.

In Galveston -- site of a 1900 hurricane that was the deadliest weather disaster in U.S. history -- lines of cars, buses and trucks crowded onto a bridge to leave the island. Others without transportation waited for buses to carry them to hurricane shelters inland after a mandatory evacuation order.

"We're getting up out of here," said Nykera Allen, a student who was loading bags into her car to drive to San Antonio. "They're going to shut the lights and the water off and that's not a good situation."

Others hunkered down to weather the storm.

"I'm just going to batten down and not worry about it," said Keith Andrews, a shipyard worker. "If the Lord wants you, he's going to take you anyway."

(Additional reporting by Anna Driver in Houston and Jim Forsyth in San Antonio; Writing by Chris Baltimore; Editing by Bill Trott)