Aircraft that were grounded due to high winds were to begin flying over stricken areas later Monday to make the first damage assessments.
Gas prices could spike because of the storm. The Energy Department reported that a dozen refineries are shut down, representing 28 percent of the Gulf Coast capacity and approximately 12 percent of U.S. refining capacity.
Another 10 refineries are operating at reduced levels.
Workers from 86 out of 171 oil rigs in the Gulf have been evacuated, and about 96 percent of oil production and 82 percent of natural gas production in the region has been temporarily shut down. If found undamaged, drilling platforms could be back in operation quickly, officials said.
About 17 percent of electric customers in Louisiana were without power, although the percentage could rise.
Houma, Morgan City and areas of Lafourche Parish were cited as places facing the greatest danger, and officials said they remained cautious.
It's "much too early" to declare success, said Maj. Gen. Don Riley, deputy commanding general of the Army Corps of Engineers. "We would not be pounding our chests."
Riley said the New Orleans levee system held even though post-Katrina reconstruction is only 25 percent complete. He said the Ninth Ward of New Orleans, wiped out in Katrina, experienced "some overwash," but added the city pumps should be able to cope with the spillover.
The levee rebuilding will not be complete until 2011, Riley said.
Harvey Johnson, deputy administrator of the Federal Emergency Management Agency, said the massive evacuations prior to the storm went well.
"All those who needed to be evacuated were evacuated," he said. But he added, "Act 2 is about to unfold," a reference to damage assessments and returning evacuees to their homes.
Maj. Gen. Bill Etter, director of domestic operations for the National Guard, said 14,000 air and army guardsmen were deployed and 50,000 were ready to respond.
The American Red Cross said 45,000 people were in its shelters, compared to 30,000 during Katrina. While the number is growing, many spaces planned as shelters have not been needed yet, said Joseph Becker, a disaster services specialist for the organization.
Earlier, emergency officials said cartons of food, water, blankets and other supplies to sustain 1 million people for three days were ready to be distributed.
FEMA has set up a registry to locate relatives. The phone number is 1-800-588-9822.
About 2 million people have been evacuated from Louisiana. Homeland Security Secretary Michael Chertoff, speaking from Baton Rouge, said he can't remember a time when FEMA was juggling so many major disasters at once.
"We do have the ability -- to use the vernacular -- to fight two wars at the same time," Chertoff told The Associated Press in an interview.
In addition to Gustav, FEMA is also dealing with Hurricane Hanna, more than a dozen major wildfires across the country, flooding in eastern and northern Florida and heavy precipitation predicted later this week over the panhandle and southern coast of Alaska.
"No doubt Hanna will present a different set of challenges," Chertoff said. "At least one bright note -- if there's any bright note in having two hurricanes -- is that it's not coming on the same location."
Forecasters predict Hanna could come ashore in Georgia and South Carolina late in the week. FEMA began preparing for Hanna several days ago and has designated a separate group of planners, FEMA's Johnson said.
"We wanted to separate them, so we'll be as ready for Hanna as we are for Gustav," Johnson said.
Johnson said FEMA has spent the last two years getting ready for the next big hurricane -- which turned out to be Gustav. Response officials in Washington and along the Gulf Coast began gearing up last week, "almost as Gustav was born, and people saw the potential size of that storm," Johnson said.
Aside from coordinating evacuation traffic, the government also spirited 3,000 residents from New Orleans by train and 5,000 by airplane, Johnson said.
"We all recall the visual images of the Coast Guard picking up people off rooftops in Katrina," Johnson said. "I don't think we'll see as much of that this year because so many people evacuated very wisely."