Residents board a bus at an evacuation center in New Orleans, Louisiana, ahead of Hurricane Gustav August 30, 2008. City officials could order a mandatory evacuation of New Orleans starting early on Sunday if Hurricane Gustav holds to its current course, New Orleans Mayor Ray Nagin said on Saturday. [Photo: Agencies]

Related: Hurricane Gustav Ravages Cuba

Spooked by predictions that Hurricane Gustav could grow into a Category 5 monster, an estimated 1 million people fled the US Gulf Coast on Saturday, even before the official order came for New Orleans residents to get out of the way of a storm taking dead aim at Louisiana.

Mayor Ray Nagin gave the mandatory order late Saturday, but all day residents took to buses, trains, planes and cars, clogging roadways leading away from New Orleans, still reeling three years after Hurricane Katrina flooded 80 percent of the city and killed about 1,600 across the region.

The evacuation of New Orleans becomes mandatory at 8 a.m. Sunday along the vulnerable west bank of the Mississippi River, and at noon on the east bank. Nagin called Gustav the storm of the century and told residents to "get your butts out of New Orleans now."

"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.

Gustav had already killed more than 80 people in the Caribbean, and if current forecasts hold up, it would make landfall Monday afternoon somewhere between the East Texas and western Mississippi.

Forecasters warned it was still too soon to say whether New Orleans would take another direct hit, but residents weren't taking any chances judging by the bumper-to-bumper traffic pouring from the city. Gas stations along interstate highways were running out of fuel, and phone circuits were jammed.

Forecasters at the National Hurricane Center said they were surprised at how quickly Gustav gained strength as it slammed into Cuba's tobacco-growing western tip. It went from a tropical storm to a Category 4 hurricane in about 24 hours, and was likely to become a Category 5, with sustained winds of 156 mph (251 kph) or more, by Sunday.

"That puts a different light on our evacuations and hopefully that will send a very clear message to the people in the Gulf Coast to really pay attention," said Federal Emergency Management Agency chief David Paulison.