The gigantic storm, which as it made landfall was roughly the size of the state of Florida, roared ashore near the coastal town of Buras, La. just after 7 a.m. EDT.
With a grim plea of "God bless us", New Orleans's mayor had ordered a mandatory evacuation of the nearly half-million residents ahead of a massive hurricane that threatened to devastate the low-lying city early Sunday.
"We are facing a storm that most of us have long feared," Mayor C. Ray Nagin said Sunday in ordering people out. "This is a once-in-a-lifetime event."
By Monday morning, Nagin told reporters that he believed some 80 percent of the city had left for higher ground.
Although the storm still poses a major threat to the city, there was good news overnight. According to the National Weather Service reported Hurricane Katrina's sustained winds, which had topped 175 mph, had died down a bit.
"Maximum sustained winds are near 135 mph with higher gusts. Katrina is an extremely dangerous category four hurricane on the Saffir-Fimpson scale," a public advisory from the National Hurricane Center reported at 9 a.m. EDT Sunday. "Weakening is forecast as the circulation interacts with land today."
The storm, which tore across southern Florida last week, killing nine and causing significant damage, has fed off the warm water of the Gulf of Mexico to become a massive storm.
"Katrina is a very large hurricane. Hurricane force winds extend outward up to 125 miles from the center and tropical storm force winds extend outward up to 230 miles," the National Hurricane Center reported Monday morning.
Despite the minimal weakening, National Weather Service officials warned the storm posed a major threat to the city.
"It would be the strongest we've had in recorded history there," National Hurricane Center Deputy Director Ed Rappaport told the Associated Press Sunday. "We're hoping of course there'll be a slight tapering off at least of the winds, but we can't plan on that. We're in for some trouble here no matter what."
For nearly 100,000 poorer residents who had no way of leaving the city, authorities opened the 70,000-seat Superdome, the home of football's Saints, Sunday. People were told to bring enough food, water and medicine to last up to five days. By Monday morning at least 10,000 people had taken shelter there.
New Orleans has not taken a major direct hit from a storm since Hurricane Betsy in 1965, when a nearly 10-foot storm surge submerged parts of the city in seven feet of water. Betsy, a Category 3 storm, was blamed for 74 deaths in Louisiana, Mississippi and Florida. Authorities have cautioned the storm surge from Katrina could top 28 feet and would likely overwhelm the city's levee system that keep the city, which lies as much as 10 feet below sea level, dry.
Although the storm ranked as a Category 4 storm when it hit shore, experts said it was likely to be one of the strongest storms to hit the United States. Only three Category 5 hurricanes have hit the United States since records have been kept. The last was 1992's Hurricane Andrew, which at 165-mph leveled parts of South Florida, killed 43 people and caused $31 billion in damage.