NEW ORLEANS -- Residents who try to ride out Hurricane Gustav will be making the biggest mistake of their lives, New Orleans Mayor Ray Nagin warned on Saturday, WDSU-TV reported.

"You need to be scared. You need to be concerned. You need to get your butts out of New Orleans. This is the storm of the century," Nagin said.

Mandatory evacuations have been ordered for Westbank starting at 8 a.m. Sunday, and mandatory evacuations of the Eastbank will begin at noon.

"Riding it out would be the biggest mistake you could make in your life," Nagin said.

Nagin warned that no emergency services will be available to residents who choose not to leave. He told residents who want to stay to make sure they have an ax, as they will be cutting through their roofs to get out of their flooded homes.

Nagin said 2005's Hurricane Katrina came ashore as a Category 3 storm. Gustav is expected to hit as either a Category 4 or 5.

On Saturday, Gustav stretched 900 miles across and it continued to grow. In contrast, the footprint of Katrina was about 400 miles in diameter when it hit.

"This storm could be just as bad -- if not worse -- than Katrina," WDSU 6 WeatherPlus meteorologist Ron Smiley said.

The announcement came as officials continued to evacuate the elderly, disabled, poor and others without means ahead of Gustav's march toward the Gulf Coast. The city estimates that 30,000 residents will need help leaving New Orleans.

"Between the buses, train and aircraft, there is no reason for anyone to ride out the storm in New Orleans. This is simply too dangerous of a storm," the Federal Emergency Management Agency's David Paulison said.

Earlier on Saturday, the National Hurricane Center called Gustav an "extremely dangerous" storm. President George W. Bush declared a state of emergency and promised full federal support to the Gulf Coast states.