New Orleans residents have been fleeing in their thousands after the city's mayor ordered an evacuation ahead of Hurricane Gustav's expected landfall.
Roads out of the Louisiana port have been crammed with traffic and authorities have been helping those unable to leave by their own means.
Mayor Ray Nagin said on Saturday the evacuation order will start with the city's low-lying West Bank starting at 8am CDT (1300 GMT) on Sunday, followed by the East Bank at noon CDT (1700 GMT).
Hurricane Gustav weakened to a Category 3 storm as it moved through the oil-rich Gulf of Mexico after hitting Cuba.
Forecasters said its path over Cuba had weakened it more than expected and the storm now had sustained winds of 201 km/h.
They said it was likely to regain strength and make landfall as a powerful Category 4 on Monday around New Orleans.
It could reach the coast of Louisiana early on Tuesday.
Vehicles are bumper to bumper on highways leading out of the city and six low-lying parishes - the Louisiana equivalent of counties - have issued mandatory evacuation orders effective later on Saturday.
All major Louisiana interstates will switch to one-way traffic away from the coast at 6am CDT (1100 GMT) on Sunday.
Tourists have been advised to leave.
The government has arranged hundreds of buses and trains to evacuate 30,000 people who cannot leave on their own.
About 1,500 Louisiana National Guard troops and 1,500 police officers are in New Orleans to oversee the evacuation.
Mr Nagin described Gustav as "the storm of the century" saying the hurricane is more powerful than Katrina.
But a spokesperson for the US National Hurricane Centre, Eric Blake, says that could be an exaggeration and right now Gustav is no Hurricane Katrina.
But he says it's still likely to be a large and powerful hurricane and a significant threat.
The mayor said he was aiming for a 100% evacuation, which extends to members of the emergency services - fewer than 50 city workers will remain in the city.
New Orleans marked the third anniversary of Hurricane Katrina on Friday.
Katrina hit the coast near New Orleans as a Category 3 hurricane on 29 August, 2005, with wind speeds up to 209km/h.
Its storm surge broke through levees and flooded 80% of the city. About 1,500 people were killed on the US Gulf Coast. Damage amounted to $US80 billion.
President George Bush declared a state of emergency in Louisiana, Mississippi, Texas and Alabama, empowering federal authorities to lead disaster relief efforts there.
Republican White House hopeful John McCain and his running-mate Sarah Palin decided to suspend their normal election campaign and visit Mississippi to inspect preparations for the arrival of Gustav.
Almost a quarter of a million people have been evacuated in Cuba, where there has been extensive flooding.
Hurricane Gustav crossed the Isle of Youth, which has 86,000 residents, before hitting the mainland in Pinar del Rio province with maximum winds of nearly 240km/h.
Heavy rains and strong winds are reported over Cuba's western province of Pinar del Rio, the main tobacco-growing region.
The US National Hurricane Center earlier said the storm was centered 135km southwest of Havana and moving northwest at 24km/h.
Power was out across most of Havana. Banana plantations have been demolished. There were no immediate reports of injuries or deaths.
Recent harvests of tobacco crops have been moved to safe places and Cuban officials said at least 200,000 people had been evacuated from the path of the storm.
In Havana, residents have been warned to take precautions.
Hurricane deaths are rare in Cuba, but Gustav killed at least 81 people in the Dominican Republic, Haiti and Jamaica. No deaths were reported in the Cayman Islands overnight.