NEW ORLEANS — With thousands feared drowned in what could be America's deadliest natural disaster in a century, New Orleans' leaders all but surrendered the streets to floodwaters Wednesday and began turning out the lights on the ruined city — perhaps for months.

Looting spiraled so out of control that Mayor Ray Nagin ordered virtually the entire police force to abandon search-and-rescue efforts and focus on the brazen packs of thieves who have turned increasingly hostile.

Nagin called for an all-out evacuation of the city's remaining residents. Asked how many people died, he said: "Minimum, hundreds. Most likely, thousands."

With most of the city under water, Army engineers struggled to plug New Orleans' breached levees with giant sandbags and concrete barriers, and authorities drew up plans to clear out the tens of thousands of remaining people and practically abandon the below-sea-level city. Most of the evacuees — including thousands now suffering in the hot and muggy Superdome — will be moved to the Astrodome in Houston, 350 miles away.

There will be a "total evacuation of the city. We have to. The city will not be functional for two or three months," Nagin said. And he said people would not be allowed back into their homes for at least a month or two.

If the mayor's death-toll estimate holds true, it would make Katrina the worst natural disaster in the United States since at least the 1906 San Francisco earthquake and fire, which have been blamed for anywhere from about 500 to 6,000 deaths. Katrina would also be the nation's deadliest hurricane since 1900, when a storm in Galveston, Texas, killed between 6,000 and 12,000 people.

In Mississippi, bodies are starting to pile up at the morgue in hard-hit Harrison County. Forty corpses have been brought to the morgue already, and officials expect the death toll in the county to climb well above 100.

President Bush flew over New Orleans and parts of Mississippi's hurricane-blasted coastline in Air Force One. Turning to his aides, he said: "It's totally wiped out. . . . It's devastating; it's got to be doubly devastating on the ground."

"We're dealing with one of the worst national disasters in our nation's history," Bush said later in a televised address from the White House, which most victims could not see because power remains out to 1 million Gulf Coast residents.

The federal government dispatched helicopters, warships and elite SEAL water-rescue teams in one of the biggest relief operations in U.S. history, aimed at plucking residents from rooftops in the last of the "golden 72 hours" rescuers say is crucial to saving lives.

'Help us!'