The city's Mayor, Ran Nagin, ordered everyone to leave because of predictions that Katrina a category 5 hurricane, the most powerful would cause extensive damage to life and property when it hit land.
"This is a once in a lifetime event. The city of New Orleans has never seen a hurricane of this magnitude hit it directly," Mr Nagin said. Those people unable to leave are to take shelter in the city's Superdome stadium, bringing enough food and supplies to last three to five days, he said.
President George Bush declared an emergency in Louisiana and Mississippi, which allows the deployment of federal emergency assistance.
Traffic jams has begun forming on main roads leading out of the city by yesterday morning, shopkeepers were busy boarding up their businesses and hotels north of the city were fully booked. Some highways had become jammed by the afternoon.
"Every person is hereby ordered to evacuate the city of New Orleans. We are facing a storm that most of us have feared. It's my hope that most people will get out," Mr Nagin said in his emergency address to the city.
The police and firefighters fanned out throughout the city telling residents to get out. Police were given the authority to commandeer any vehicle or building that could be used for evacuation or shelter.
The US National Hurricane Centre in Miami, Florida, said that Katrina, which has already killed seven people and caused extensive devastation in Florida, has strengthened to category 5, the highest on the hurricane scale, with wind speeds of up to 175 mph (280 kph).
New Orleans is particularly vulnerable to hurricanes because most of the city lies below sea level and so is liable to suffer extensive flooding. Storm surges could easily breach the levees or dams that protect the city from the surrounding water of Lake Pontchartrain, the Mississippi river and nearby marshes.
The hurricane is calculated to have the potential for storm-surge flooding of up to 25 feet, topped with even higher waves, as much as 15 inches of rain, and tornadoes, the National Hurricane Centre warned.
Ed Rappaport, the deputy director of the centre, said that Katrina could still hit land in a number of places, including Alabama and Florida, although the prediction was that it would reach New Orleans later today.
"If it came ashore with the intensity it has now and went to the New Orleans area, it would be the strongest we've had in recorded history," he said.
"We're hoping there'll be a slight tapering off, at least of the winds, but we can't plan on that. So whichever area gets hit, this is going to be a once-in-a-lifetime event for them," he said.
Category 5 hurricanes are as strong as they come and can bring down buildings and power lines, as well as causing extensive flooding. Deaths were almost inevitable, Mr Rappaport said. "We're in for some trouble here no matter what."
Katrina was 225 miles south-south-east of the Mississippi delta last night and heading north-west at 12 mph. Hurricane force winds could be felt 105 miles from the eye of the storm.
The last category 5 to strike the area was Hurricane Camille in 1969. Camille just missed New Orleans but devastated large swaths of Louisiana and Alabama, and killed hundreds of people.
Hurricane Andrew, which destroyed the city of Homestead in southern, Florida in 1992 and ranks as the costliest natural disaster in US history, was also a category 5 storm.Reuse content