Monstrous Hurricane Katrina barreled toward New Orleans, U.S. on Sunday with 165-mph winds and a threat of a 28-foot storm surge, forcing a mandatory evacuation, a last-ditch Superdome shelter and prayers for those left to face the doomsday scenario this below-sea-level city has long dreaded.

"Have God on your side, definitely have God on your side," Nancy Noble said as she sat with her puppy and three friends in six lanes of one-way traffic on gridlocked Interstate 10. "It's very frightening."

Katrina intensified into a Category 5 giant over the warm water of the Gulf of Mexico on a path to come ashore early Monday in the heart of New Orleans. That would make it the city's first direct hit in 40 years and the most powerful storm ever to slam the city.

"I'm really scared," resident Linda Young said as she fill her gas tank. "I've been through hurricanes, but this one scares me. I think everybody needs to get out."

Rain began falling on southeastern Louisiana by midday Sunday, the first hints of a storm with a potential surge of 18 to 28 feet, topped with even higher waves, tornadoes and as much as 15 inches of rain.

"We are facing a storm that most of us have long feared," Mayor Ray Nagin said in ordering the mandatory evacuation for his city of 485,000 people, surrounded by suburbs of a million more. "The storm surge will most likely topple our levee system."

Conceding that as many as 100,000 inner-city residents didn't have the means to leave and an untold number of tourists were stranded by the closing of the airport, the city arranged buses to take people to 10 last-resort shelters, including the Superdome.

Nagin also dispatched police and firefighters to rouse people out with sirens and bullhorns, and even gave them the authority to commandeer vehicles to aid in the evacuation.

"This is very serious, of the highest nature," the mayor said. "This is a once-in-a-lifetime event."

For years, forecasters have warned of the nightmare scenario a big storm could bring to New Orleans, a bowl of a city that's up to 10 feet below sea level in spots and dependent on a network of levees, canals and pumps to keep dry. It's built between the half-mile-wide Mississippi River and Lake Pontchartrain, half the size of the state of Rhode Island.

Estimates have been made of tens of thousands of deaths from flooding that could overrun the levees and turn New Orleans into a 30-foot-deep toxic lake filled with chemicals and petroleum from refineries, and waste from ruined septic systems.

Katrina's eye was expected to make landfall around sunrise Monday on the southeastern Louisiana coast, although Mississippi also was in danger, said Ed Rappaport, deputy director of the National Hurricane Center in Miami. Because Katrina was such a big storm with hurricane-force wind of at least 74 mph extending up to 105 miles from the center, areas far from the eye's landfall could still be devastated.

At 5 p.m. ET, Katrina's eye was about 150 miles south of the mouth of the Mississippi River. The storm was moving toward the northwest at nearly 13 mph and was expected to turn toward the north-northwest.

A hurricane warning was in effect for the north-central Gulf Coast from Morgan City, La., to the Alabama-Florida line, the hurricane center said. Tropical storm warnings extended east to Indian Pass, Fla., and west to Cameron, La., a spread of about 480 miles.

Source: Agencies