NEW ORLEANS — Spooked by predictions that Hurricane Gustav could grow into a Category 5 monster, an estimated 1 million people fled the Gulf Coast yesterday — 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 yesterday, but all day residents had been taking 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 people across the region.

The evacuation of New Orleans becomes mandatory at 8 a.m. today along the vulnerable west bank of the Mississippi River, and at noon on the east bank.

Nagin called Gustav the "mother of all storms" and told residents to "get out of town. This is not the one to play with."

"This is the real deal, this is not a test," Nagin said as he issued the order, warning residents that staying would be "one of the biggest mistakes you could make in your life."

He emphasized that the city will not offer emergency services to anyone who chooses to stay behind.

Nagin did not immediately order a curfew, a move that would allow officials to arrest residents who are not on their own property.

Gustav has already killed more than 80 people in the Caribbean, and was predicted to make landfall tomorrow afternoon somewhere between east Texas and western Mississippi.

The storm's center moved into the Gulf of Mexico from Cuba late yesterday, and at 2 a.m. EDT (8 p.m. Hawai'i time) was about 485 miles southeast of the mouth of the Mississippi River, the National Hurricane Center in Miami said.

Top winds were near 135 mph and likely to strengthen.

Forecasters warned it was 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 or more — by today.

"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 David Paulison, chief of the Federal Emergency Management Agency.

Levee building on the city's west bank is incomplete, Nagin said. A storm surge of 15 to 20 feet would pour through canals and flood the neighborhood there and neighboring Jefferson Parish, he said.

As part of the evacuation plan New Orleans developed after Katrina, residents who had no other way to get out of the city waited for buses in a line that snaked for more than a mile through the parking lot of the city's main transit terminal. From there, they were boarding motor coaches bound for shelters in north Louisiana.

Forecasters said Gustav was just short of becoming a top-scale Category 5 hurricane when it hit Cuba. At least 300,000 people were evacuated from the storm's path in Cuba.

On Isla de la Juventud (Isle of Youth), a Cuban island of 87,000 people south of the main island of Cuba, Gustav's screaming 140 mph winds toppled telephone poles and mango and almond trees, and peeled back the tin roofs of homes.

Civil defense chief Ana Isla said there were "many people injured" on Isla de la Juventud, but no reports of deaths. She said nearly all of the island's roads were washed out and that some areas were heavily flooded."It's been very difficult here," she said on state television.