Residents fled a partially rebuilt New Orleans Sunday as a monster storm churning in the Gulf of Mexico caused a mandatory evacuation, nearly three years to the day after Hurricane Katrina wiped out entire swaths of the city.

Hurricane Gustav slowed to Category 3 status with top winds near 125 mph early Sunday but forecasters expected it to regain strength later in the day. The National Hurricane Center upgraded a watch to a hurricane warning for over 500 miles of Gulf coast from Cameron, La., near the Texas border to the Alabama-Florida state line, meaning hurricane conditions are expected there within 24 hours.

The storm was projected to make landfall as early as Monday, and could bring a storm surge of up to 20 feet to the coast and rainfall totals of up to 15 inches.

Gustav, which already killed more than 80 people in the Caribbean, strengthened quickly into a Category 4 storm Saturday. It slammed Cuba's tobacco-growing western tip before moving away from the island country into the Gulf of Mexico.

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New Orleans Mayor Ray Nagin ordered the mandatory evacuation of New Orleans, while Louisiana Gov. Bobby Jindal warned residents that the state "could see flooding worse than Hurricane Katrina," as Gustav approached the Gulf Coast.

Nagin used stark language to urge residents to get out of the city, calling Gustav the "storm of the century."

"This is the real deal, not a test," Nagin said as he issued the evacuation order Saturday night. "For everyone thinking they can ride this storm out, I have news for you: that will be one of the biggest mistakes you can make in your life."

Even residents in the city's western suburb of Jefferson Parish, which was swollen by residents who did not return to New Orleans after Katrina, were ordered to leave. It was the first mandatory evacuation for the entire parish.

In Alabama, Gov. Bob Riley issued a mandatory evacuation order for some coastal areas of Mobile and Baldwin counties, effective at 7 a.m., in response to Hurricane Gustav.

Forecasters were slightly less dire in their predictions, saying the storm should make landfall Monday afternoon somewhere between western Mississippi and East Texas, where evacuations were also under way. It's too early to know whether New Orleans will take another direct hit, they said, but city officials weren't taking any chances.

Many residents didn't need to be ordered, with an estimated 1 million people fleeing the Gulf Coast on Saturday by bus, train, plane and car. They clogged roadways, emptied gas stations of fuel and jammed phone circuits.

Gustav's center was about 425 miles southeast of the Mississippi River's mouth at 5 a.m. EDT as it moved northwest near 16 mph.

The mandatory evacuation of the city's west bank, where levee improvements remain incomplete, was to begin at 8 a.m., with the east bank to follow at noon. It's the first test of a revamped evacuation plan designed to eliminate the chaos, looting and death that followed Katrina.

The city will not offer emergency services to those who choose stay behind, Nagin said, and there will be no "last resort" shelter as there was during Katrina, when thousands suffered inside a squalid Superdome. The city said in a news release that those not on their property after the mandatory evacuation started would be subject to arrest.

Many residents didn't need to be ordered, with an estimated 1 million people fleeing the Gulf Coast on Saturday by bus, train, plane and car. They clogged roadways, emptied gas stations of fuel and jammed phone circuits.

At the city's main transit terminal, a line snaked through the parking lot for more than a mile as residents with no other means of getting out waited to board buses bound for shelters in north Louisiana and beyond.

"I'm not staying for 'em any more," said Lester Harris, a 53-year-old electrician waiting at a bus pickup point in the Lower 9th Ward. He was rescued from his house by boat after Katrina. "I got caught in the water and spent two days on my roof. No food, no water. It was pretty bad."

Mike Mayer, owner of Jefferson Indoor Range and Gun Outlet in suburban Metairie, said sales of guns and ammunition were up.

"My business doubled," he said. "People are afraid of coming back after the storm. ... They want some protection when they walk back in."

Homeland Security Secretary Michael Chertoff planned to travel to Louisiana on Sunday to observe preparations. And likely GOP presidential nominee John McCain and his running mate, Alaska Gov. Sarah Palin, were traveling to Mississippi.

Despite the stern warnings from Nagin and others, the expected arrival of 2,000 National Guard troops suggested officials were expecting stragglers.

Stephen Sonnier left for Katrina, but not this time.

"I'll never leave again. Just being away, worrying about it last time? I'd have rather been here," Sonnier said as he helped his friend Bill Espy use an electric drill to fasten metal hurricane panels over the window of his reconstructed flower shop.

Sonnier had just marked the third anniversary of Katrina on Friday by placing flowers on a makeshift memorial to a woman named Vera who was struck by a car after the storm. Her body lay unattended for days before neighbors built a makeshift brick tomb around her. Pictures of that grave with its spray-painted epitaph: "Here lies Vera, God Help Us!" became one of the symbols of the post-Katrina mayhem.

Many residents said the early stage of the evacuation was more orderly than Katrina, although a plan to electronically log and track evacuees with a bar code system failed and was aborted to keep the buses moving. Officials said information on evacuees would be taken when they reached their destinations.

Some began arriving Saturday in Arkansas, where the National Guard prepared to shelter thousands for weeks. At least 15,000 people sought refuge in the inland state in 2005, following Katrina and Rita.

Meanwhile, as many as 500 critical-care patients were being airlifted from hospitals along the Gulf Coast to Dallas-Fort Worth International Airport, a spokesman said. The patients were being taken to about 20 hospitals around North Texas.

Traffic late Saturday night was stop and go on Interstate 10, heading west into Houston from the Louisiana border, as Texas prepared to house up to 45,000 evacuees, even though that state's eastern stretches were within the range of where Gustav could make landfall.

In Beaumont, not far from where Hurricane Rita roared ashore as a Category 3 in 2005, residents were boarding up homes and leaving. In neighboring Orange County, officials were inundated "by thousands" of people calling to register for evacuation assistance, a county spokeswoman said.

To the east, Louisiana residents were checking into hotels along Alabama's coast. Mitch and Laura Tucker of Mandeville brought along their dog, Roux, whom they saved during Katrina.

"We don't know what we'll be going back to," he said.

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The Associated Press contributed to this report.