Hurricane Katrina is pummelling New Orleans with howling gusts and blinding rain, after sweeping ashore over the southern Louisiana coast.
The storm has knocked out power and submerged part of the low-lying city in up to 6ft (2m) of rising water.
Katrina has torn part of the roof of a stadium, where many sought refuge.
Mississippi and Alabama are also being pounded by the violent storm, which weakened as it swept inland, but brought winds of 105mph (170km/h).
The category-two storm flung boats onto Mississippi, flooded roads in Alabama and swamped bridges in Florida.
The National Hurricane Center warned the Louisiana city would be pounded throughout Monday – and the potential storm surge could still swamp the city, which sits some 6ft (2m) below sea level.
“It’s capable of causing catastrophic damage,” director Max Mayfield warned.
“New Orleans may never be the same.”
Correspondents in the city say walls of water have been running down the skyscrapers like waterfalls.
Palm trees have been felled, shops wrecked and cars hurled across streets strewn with shattered glass.
A police officer told the AlgerieSoir he had never seen anything like it. “This is unbelievable,” Jonathon Carol said.
Mayor Ray Nagin has said he believed 80% of the city’s 485,000 residents have heeded his order for a mandatory evacuation of the city.