The mayor of New Orleans ordered the immediate evacuation of the city today as Hurricane Katrina, now a Category 5 storm packing 175 mph winds, bore down on the Louisiana coast after gathering strength in the Gulf of Mexico.

Mayor Ray Nagin issued the mandatory evacuation order amid fears that the hurricane could cause massive flooding in New Orleans, a city of 485,000 people that lies below sea level.

Appearing on live television at a news conference to issue the order, Nagin said authorities were setting up 10 refuges of last resort, including the city's Superdome, for people who were unable to get out. He said the hurricane's storm surge was likely to overwhelm the levees that protect the city.

Louisiana Gov. Kathleen Blanco, who joined Nagin at the news conference, said President Bush had called her moments before to urge a mandatory evacuation.

"There doesn't seem to be any relief in sight," Blanco said. She pointed out that even when Katrina hit Florida last week as a Category 1 storm, well below its current strength, it killed half a dozen people.

"The city of New Orleans has never seen a hurricane of this magnitude hit it directly," Nagin said. Calling Katrina a "once-in-a-lifetime event," he said, "We are facing a storm that most of us have long feared."

The National Weather Service this morning declared Katrina "a potentially catastrophic" hurricane after it strengthened to a Category 5 storm in the Gulf. The storm sliced across the southern tip of Florida from the Atlantic before heading westward into the Gulf, where it was fueled by the warm Gulf waters and turned to the north.

Bush declared states of emergency in Louisiana and Mississippi to facilitate a federal emergency response. The Federal Emergency Management Agency was sending water, food and other supplies to staging centers in the Southeast for distribution to areas hit by Katrina.

Mayor Nagin said New Orleans police and firefighters would fan out throughout the city telling residents to get out and that police would have the authority to commandeer any vehicle or building that could be used for evacuation or shelter, the Associated Press reported. The mayor said people who opted to go to the Superdome should take enough food and supplies to last three to five days. He said hotels were exempted from the evacuation order because airlines had already canceled all flights out.

The weather service issued a hurricane warning for the north-central Gulf Coast from Morgan City, La., eastward to the Alabama-Florida border. It said that as of 11 a.m. EDT, Katrina was 225 miles south-southeast of the mouth of the Mississippi River and moving west-northwest at about 12 mph.

Regarding New Orleans specifically, the weather service said it expected a "direct strike" by Katrina with "potentially catastrophic and life-threatening" consequences. It urged people to "rush protective measures to completion and leave the area now!"

Local meteorologists predict that Katrina will make landfall at around 7 a.m. Monday morning and that it will likely run smack into New Orleans.

The weather service said reports from an Air Force hurricane hunter aircraft indicated that Katrina's maximum sustained winds increased to near 175 mph, from about 160 mph reported earlier this morning, and that it was packing even higher gusts.

"Hurricane force winds extend outward up to 105 miles from the center, and tropical storm force winds extend outward up to 205 miles," the weather service said.

It predicted coastal storm surge flooding of 18 to 22 feet above normal tide levels and locally as high as 28 feet, with "large and dangerous battering waves" near the center of the landfall and to the east.

It said Katrina was likely to dump 5 to 10 inches of rain, with "isolated maximum amounts of 15 inches," along the storm's path across the Gulf Coast and the Tennessee Valley.

"Isolated tornadoes will be possible beginning this evening over southern portions of Louisiana, Mississippi and Alabama, and over the Florida Panhandle," the weather service said.