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.
The announcement turned informal advice to flee from Gustav into an official order to get out.
Nagin said Saturday that the evacuation becomes mandatory at 8 a.m. Sunday on the city's vulnerable west bank. It becomes mandatory on the east bank at noon.
"This is the real deal, not a test," Nagin said as he issued the order, warning residents that staying would be "one of the biggest mistakes of your life." He emphasized that the city will not offer emergency services to anyone who chooses to stay behind.
The announcement comes as officials continued to evacuate the elderly, disabled, poor and others without means ahead of Gustav's march toward the Gulf Coast.
"[This is] as bad as it gets," the Republican governor said, quoting the National Weather Service.
An estimated 1 million residents fled the Gulf Coast Saturday, ahead of any official evacuation order, according to the Associated Press, but after the National Hurricane Center issued a hurricane watch for Alabama, Mississippi, Louisiana and part of Texas.
The watch area includes New Orleans, where residents marked the third anniversary of Hurricane Katrina's devastation on Friday. The watch stretches from east of High Island, Texas, to the Alabama-Florida border.
Hurricane Gustav strengthened to a dangerous Category 4 storm Saturday, prompting officials to plan a special advisory and some Gulf Coast residents to leave town ahead of mandatory evacuations.
Data from an Air Force reconnaissance aircraft indicated that Gustav's maximum winds have increased to close to 145 mph, making the already-deadly storm an extremely hazardous Category 4 hurricane on the Saffir-Simpson hurricane scale.
Gustav could strengthen even further to a Category 5 hurricane just before or shortly after it passes over Cuba into the Gulf of Mexico, according to the National Hurricane Center.
New Orleans residents, with the memory of devastating Hurricane Katrina still fresh in their minds, didn't waste time getting out of the city on Saturday.
Lines of people waiting for buses to take them out of the Big Easy grew longer and traffic grew heavier on main highways as Hurricane Gustav strengthened into a dangerous storm on track for the Gulf Coast.
"I'm getting out of here. I can't take another hurricane," said Ramona Summers, 59, whose house flooded during Katrina. She hurried to help friends gather their belongings. Her car was already packed for Gonzales, nearly 60 miles away to the west of New Orleans.
New Orleans Mayor Ray Nagin said a mandatory evacuation hadn't yet been issued, but could come later in the day to begin Sunday morning.
He said he was confident the levees wouldn't fail the way they did during Katrina if Gustav hits New Orleans.
"We have invested a significant amount of money in levee protection, so I’m not anticipating the same type of levee flooding we had with Katrina," Nagin told reporters Saturday. He said he was expecting street flooding, and city officials would be working through the night in preparation for the monster storm.
As for whether or not New Orleans could survive another devastating hurricane, Nagin said it would be difficult.
"Emotionally can we handle it? I think there's a lot of fragileness in the city," he said. "It’s going to be a tough one, but New Orleanians are very resilient."
A line well over a mile long stretched in six loops through the parking lot at Union Station Terminal in New Orleans. Under a blazing sun, many led children or pushed strollers with one hand and pulled luggage with the other. Volunteers handed out bottled water, and medics were nearby in case people became heatsick.
The Federal Emergency Management Agency (FEMA) was already working feverishly to avoid the kind of problems that surfaced in the aftermath of Hurricane Katrina. FEMA said it expects a "huge number" of Gulf Coast residents will be told to leave the region this weekend.
Joseph Jones Jr., 61, wore a towel over his head to block the sun. He'd been in line 2 1/2 hours, but wasn't complaining. During Katrina, he had been stranded on a highway overpass.
"I don't like it. Going someplace you don't know, people you don't know," Jones said. "And then when you come back, is your house going to be OK?"
The city had yet to call for a mandatory evacuation, but began ushering out the sick, elderly and those without their own transportation on Saturday. The state has a $7 million contract for more than 700 buses to carry an estimated 30,000 people to shelters.
Many residents said the evacuation was more orderly than Hurricane Katrina, which struck three years ago Friday. But not everyone was happy.
Elizabeth Tell, 67, had been waiting on the corner since 6:30 a.m. for a special needs bus to take her and her dog, Lee Roy, to the station. It was three hours before the first bus arrived, completely full of people in wheelchairs.
"They're not taking care of us down here!" she shouted as the brown-and-white spotted hound mix panted inside his hip-high plastic kennel.
Many residents weren't waiting for a formal evacuation call. Cars packed with clothes, boxes and pet carriers drove north among heavy traffic on Interstate 55, a major route out of the city. Gas stations around the city hummed. And nursing homes and hospitals began sending patients farther inland.
Police and firefighters were set to go street-to-street with bullhorns over the weekend to help direct people where to go. Unlike Hurricane Katrina, there will be no shelter of last resort in the Superdome. The doors there will be locked.
Those among New Orleans' estimated 310,000 to 340,000 residents who ignore orders to leave accept "all responsibility for themselves and their loved ones," the city's emergency preparedness director, Jerry Sneed, has warned.
There were signs Saturday morning of people racheting up their plans to leave. ATM machines were running out of cash. Long lines were sprouting up at gas stations as motorists filled up their cars. Cases of bottled water were selling briskly at convenience stores.
Gustav swelled into a major hurricane south of Cuba and could strike the U.S. coast anywhere from Mississippi to Texas by Tuesday.
Forecasters said if Gustav follows the projected path it would likely make landfall on Louisiana's central coast, sparing New Orleans a direct hit. But forecasters caution it is still too soon to say exactly where the storm will hit.
"Any little jog could change where it makes landfall," said Karina Castillo, a hurricane support meteorologist at the National Hurricane Center said.
One shop along Magazine Street, its windows covered up, showed a flash of New Orleans' storm humor. "Geaux Away Gustav," it read, giving it a French flair.
President Bush called Gulf Coast governors Saturday and told them they would have the full support of the federal government, White House spokesman Scott Stanzel said.
Officials plan to announce a curfew that will mean the arrest of anyone still on the streets after a mandatory evacuation order goes out. Police and National Guardsman will patrol after the storm's arrival, and Gov. Bobby Jindal has said he requested additional search and rescue teams from other states.
Jindal also said the state would likely switch interstate lanes on Sunday so that all traffic would flow north, in the direction an evacuation would follow.
By midday Saturday, Gustav was about 185 miles east of the western tip of Cuba and just 55 miles east-southeast of the Isla de Juventud. It was expected to be moving northwest near 14 mph.
Hurricane force winds extended out 60 miles in some places.
Cuban officials raced to evacuate more than 240,000 people
The center of Gustav was to pass over western Cuba later Saturday and strengthen is forecast after it reaches the Gulf of Mexico.
The second major hurricane of the Atlantic season has already killed 78 people in the Caribbean.
Haiti's Interior Ministry on Saturday raised the hurricane death toll there to 66 from 59. Gustav also killed eight people in the Dominican Republic and four in Jamaica.
Cuba grounded all national airline fights, though planes bound for international destinations were still taking off at Havana's Jose Marti International Airport.
Authorities also canceled all buses and trains to and from the capital, as well as ferry and air service to the Isla de Juventud, the outlying Cuban island-province next in Gustav's path.
The U.S. naval base at Guantanamo, Cuba, was hundreds of miles to the east, out of the storm's path.
Gustav could strike the U.S. Gulf coast anywhere from the Florida Panhandle to Texas, but forecasters said there was an increasing chance that New Orleans will get slammed by at least tropical-storm-force winds, three years after devastating Hurricane Katrina.
As much as 80 percent of the Gulf of Mexico's oil and gas production could be shut down as a precaution if Gustav enters as a major storm, weather research firm Planalytics predicted. Oil companies have already evacuated hundreds of workers from offshore platforms.
The Associated Press contributed to this report.