Thousands of homes were flooded, roads were washed out and several fires burned uncontrolled as emergency teams were ordered out of the area.
The US National Weather Service warned that people in coastal areas could "face the possibility of death" if they try to ride out the storm.
Michael Chertoff, the US homeland security secretary, said: "Our nation is facing what is by any means a potentially catastrophic hurricane.
"This certainly falls in the category of pretty much a worst-case scenario."
Officials feared that thousands of people had defied evacuation orders and would need rescuing from flood-hit homes.
Andrew Barlow, spokesman for the Texas governor's offices, said: "The unfortunate truth is we're going to have to go in ... and put our people in the tough situation to save people who did not choose wisely. We'll probably do the largest search and rescue operation that's ever been conducted in the state of Texas."
Authorities in three counties alone said roughly 90,000 stayed behind.
Ports were closed and the US Coast Guard said a ship with 22 people aboard was stranded without power 145km off the US coast, although a tugboat was on the way to save them.
The US federal emergency management agency said more than 5.5 million prepackaged meals were being sent to the region, along with more than 230 generators and 5.6 million litres of water.
The storm is on course to pass directly over Houston - the fourth-largest city in the US.
|Hurricane Ike comes in the midst of a heavy storm season [AFP]
Al Jazeera's Mike Kirsch, reporting from Houston, said the storm was expected to affect 400km of the southern US coastline.
About three million people were reported to be without electricity in the Houston area and suppliers warned it could be weeks before services were fully restored.
In central Houston, about 80km inland from Galveston, businesses closed and windows were boarded up.
An overnight curfew was also imposed in evacuated areas of the city to prevent looting.
George Bush, the US president, said he was "deeply concerned" about the storm and for those in Texas, his home state.
The storm was also expected to cause further disruption to the oil industry, with 17 refineries in Texas already forced to shut down.
"Close to 20 per cent of the US refining capability could be lost for a long period of time," Jim Rouiller, a meteorologist at Planalytics, a private weather forecaster, said.
Ike comes in the midst of a heavy storm season in the US and Caribbean and just 10 days after hurricane Gustav struck the US coast.
Thunderstorms shut down schools on Friday and knocked out power throughout southern Louisiana state, while around 1,200 people sought refuge in government shelters.
The hurricane has already battered the Caribbean, causing at least four deaths in Cuba, where about 2.6 million people were evacuated, and thousands of buildings and crops damaged.