New Orleans Mayor Ray Nagin ordered the city's more than 239,000 residents to evacuate on Sunday in the face of powerful Hurricane Gustav, which he called "the mother of all storms."
The evacuation order issued on Saturday was the first in New Orleans since Hurricane Katrina devastated the historic Southern city in August 2005.
"This is the mother of all storms," Nagin said of Gustav, a monstrous Category 4 storm that could approach the central Louisiana coast just west of New Orleans on Monday.
"You need to be concerned and you need to get your butts moving and out of New Orleans right now," Nagin said at City Hall. "This is the storm of the century."
The evacuation order, which will not be physically enforced by officials, will start with the city's low-lying West Bank starting at 8 a.m on Sunday, followed by the East Bank at noon, Nagin told reporters.
Residents have the choice to remain behind and weather the storm, but "that would be one of the biggest mistakes that you could make in your life," Nagin said.
He said people might have to chop through the roofs of their houses to escape rising waters if they stay.
"Make sure you have an ax," he said.
But one day after the third anniversary of Katrina, many had already decided to abandon the city, much of which lies below sea level.
Thousands of people fled New Orleans earlier on Saturday. Hoping to avoid the 2005 spectacle of desperate city residents crammed into the New Orleans Superdome, the government lined up hundreds of buses and trains to evacuate 30,000 people who cannot leave on their own.
About 10,000 people left the city by bus or train on Saturday, Nagin said. The rest of the 20,000 people that had requested evacuation assistance would leave on Sunday, he added.
Many evacuees were issued wrist bands with bar codes that will allow city officials to track them.
Gustav crashed across the Cuban mainland on Saturday and could hit the U.S. Gulf Coast as a Category 4 storm, the second-highest on the five-stage Saffir-Simpson scale, the U.S. National Hurricane Center said.
SIGNIFICANT FLOODING EXPECTED
In the Lower Ninth Ward, plunged under water by Katrina's floodwaters, hundreds of residents packed belongings into cars and trucks and left. Some had returned home only a few months ago after fleeing Katrina.
"After Katrina, you've got to leave," said Ruby Hall, a longtime resident, pointing to the place on the timber frame of the porch where Katrina's waters rose. "I'm not going to chance it, not with my grandchild."
The city's West Bank was largely spared by Katrina but could see "significant flooding" because its 10-foot (3-metre) levees are no match for Gustav's storm surge, which could top 20 feet (6 metres), Nagin said.
Katrina's massive storm surge broke through protective levees on Aug. 29, 2005, and flooded 80 percent of the city. New Orleans degenerated into chaos as stranded storm victims waited days for rescue.
The hurricane killed about 1,500 people along the U.S. Gulf Coast and caused $80 billion in damages, making it the costliest U.S. natural disaster.
There was bumper-to-bumper traffic on highways leading out of the city on Saturday, and six low-lying parishes -- the Louisiana equivalent of U.S. counties -- issued evacuation orders.
All major Louisiana interstates will allow only one-way traffic away from the coast starting at 4 a.m. CDT (0900 GMT) on Sunday. The last flight out of the New Orleans airport is scheduled to depart at 6 p.m. CDT (2300 GMT) on Sunday.
In all, 11.5 million people are in the path of Gustav, according to the U.S. Census Bureau.
Walter Parker, a security guard who was trapped for eight days in his apartment during the Katrina flooding, lined up outside the Union Passenger Terminal as families with bags packed and children in tow waited for transportation.
"I don't want to see another Katrina, with dead bodies floating in the water," Parker said.