"This is the real deal, not a test," Nagin said as he issued the order, effective 9 a.m. Eastern time Sunday for low-lying areas and 1 p.m. citywide. He warned residents that staying would be "one of the biggest mistakes of your life."
Forecasters warned that it was still too soon to say whether New Orleans would take a direct hit from Gustav late Monday, but the storm's threat, coming three years after Hurricanes Katrina and Rita devastated a broad swath of the Gulf Coast, drew a hefty amount of wary respect from city, state and federal officials.
Gustav has already killed more than 80 people in the Caribbean. On Saturday, it slammed into western Cuba, knocking out power in Havana. The Cuban government said that it had moved at least 300,000 people.
In New Orleans, local officials said they would turn all lanes of traffic on major highways into one-way routes headed away from the city, starting early Sunday morning.
But many residents were not waiting to leave. At a news conference at 9:30 p.m. Eastern time Saturday, Nagin said 50 percent of the city had already evacuated.
By dinnertime, St. Charles Avenue, the main drag through the residential Garden District, was all but deserted. National Guard troops patrolled the street, walking by a few celebrants of Southern Decadence, an annual Labor Day weekend event that draws thousands of gays and lesbians.
Jackson Square, a part of the French Quarter that is regularly lined with horse-drawn carriages and street artists, was abandoned as well, save for a few palm readers and homeless people. Private security guards wearing bulletproof vests and carrying semiautomatic weapons were out in force in front of the InterContinental Hotel, which was preparing to evacuate all guests and close its doors Sunday morning.
Under a worst-case scenario, Gustav could "put the whole city under" water, Nagin said, even areas that have never flooded before. "This is the mother of all storms," he said.
The hurricane also threatened to disrupt oil and natural gas production in the Gulf of Mexico, energy analysts warned, and companies with offshore rigs in the gulf said they had significantly cut their production. Oil refiners also reduced their operations.
After clearing the Cayman Islands, Gustav gained strength Saturday and became a Category 4 storm on the Saffir-Simpson scale. Forecasters are predicting that the storm will reach Category 5 -- the strongest level -- with winds higher than 155 mph before hitting the Gulf Coast. The National Hurricane Center issued a hurricane watch for Alabama, Mississippi, Louisiana and Texas's eastern coast.
At Union Passenger Terminal in New Orleans, the city's Amtrak station and one of 17 evacuation centers, residents said they were wiser about the danger of Gustav after going through the ravages of Katrina.