News Flash
New Orleans is mostly below sea level and protected by levees or embankments.
Nagin said the levees had given way in places to Katrina's storm surge, including a 200-foot (60 meter) breach near the city center through which waters from Lake Pontchartrain were pouring in.
"There's a serious leak and it's causing the water to continue to rise," he said. Adding to the problem were malfunctions in the system the city uses to pump out floodwaters.
So far, Nagin said, the historic French Quarter and central business district had not been badly flooded.
But Tulane University Medical Center vice president Karen Troyer-Caraway told CNN the downtown hospital was surrounded by 6 feet of water and considering evacuating its 1,000 patients.
"The water is rising so fast I cannot begin to describe how quickly it's rising," she said. "We have whitecaps on Canal Street, the water is moving so fast."
Louisiana emergency-preparedness officials said plans were in the works to fix the broken levee.
The high waters flooded thousands of homes and forced many people into attics and onto roofs.
"HORROR STORY"
Police took boats into flood-stricken areas to rescue some of the stranded. Others were picked up by helicopter.
People used axes, and in at least one case a shotgun, to blast holes in roofs so they could escape their attics. Many who had not yet been rescued could be heard screaming for help, police said.
"This is a horror story. I'd rather be reading it somewhere else than living it," said Aaron Broussard, president of New Orleans' Jefferson Parish.
In Mississippi, water swamped the emergency operations center at Hancock County courthouse and the back of the building collapsed.
"Thirty-five people swam out of their emergency operations center with life jackets on," neighboring Harrison County emergency medical services director Christopher Cirillo told Mississippi's Sun Herald newspaper. "We haven't heard from them."
The storm revived memories of Hurricane Camille, which hit the region in 1969 with winds up to 200 mph (320 kph) and killed 256 people.
Before striking the Gulf coast, Katrina last week hit southern Florida, where it killed seven people.
Katrina knocked out electricity to about 2.3 million customers, or nearly 5 million people, in Louisiana, Mississippi, Alabama and Florida, utility companies said. Restoring power could take weeks, they warned.
On its way to the coast, the storm swept through oil and gas fields in the Gulf of Mexico where 20 percent of the nation's energy is produced.
At least two drilling rigs were knocked adrift and one in Mobile Bay, Alabama, broke free of its mooring and slammed into a bridge.
