By noon, residents in the rest of the city were supposed to be out of their homes and heading to safety.

It was projected to make landfall as early as Monday, and could bring a storm surge of up to 6 metres to the coast and rainfall totals of up to 38 centimetres.

Mayor Ray Nagin called Gustav 'the mother of all storm', and says anyone ignoring calls to leave would be on their own.

He estimated that less than half of the city's population has left despite days of dire warnings.

'This is the real deal,' he said. 'Riding this storm out would be one of the biggest mistakes you could make in your life.'

Mr Nagin said police, fire and other emergency personnel are being pulled from the city to safer areas. A 'skeleton crew' of fewer than 50 city workers will be left behind, according to officials.

Hurricane Gustav, which has been downgraded to a Category Three storm, is on course to crash ashore near New Orleans.

Mr Nagin told anyone planning to stay behind to 'make sure you have an axe because you will be busting your way out to get on your roof with waters surrounding you'.

Early on Saturday, thousands of people lined up outside a bus and train terminal to get out of New Orleans as Hurricane Gustav took aim at the Louisiana coast, reviving traumatic memories of Hurricane Katrina.

The city, which marked the third anniversary of Katrina on Friday, had not yet issued a mandatory evacuation order. City Mayor Ray Nagin said if Gustav holds to its current course, a mandatory city evacuation could start early on Sunday.

But with memories of Katrina and its devastation still fresh, many already had decided to abandon the city, much of which lies below sea level. Gustav was heading towards western Cuba on Saturday, and could reach the Louisiana coast early on Tuesday.

Cars crammed bumper to bumper on highways leading out of the city and six low-lying parishes - the Louisiana equivalent of US counties - issued mandatory evacuation orders effective later on Saturday. All major Louisiana interstates will switch to allow only one-way traffic away from the coast at 6 am CDT (7pm Singapore time) on Sunday.

Hoping to avoid the 2005 spectacle of desperate city residents crammed into the New Orleans Superdome, the government has lined up 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 - twice the level seen during Katrina.

Memories of floating bodies
Thousands lined up to board buses, with no knowledge of their eventual destination. So far, about 1,200 people have left the city by bus, and another 1,500 by train, Mr Nagin said, with about 20,000 people requesting evacuation assistance.

Many evacuees were issued wrist bands with bar codes that will allow city officials to track them.

Mr Walter Parker, a security guard who was trapped for eight days in his apartment during the Katrina flooding, lined up outside the Union Passenger Terminal as families with bags packed and children in tow waited for transportation.

'I don't want to see another Katrina, with dead bodies floating in the water,' Mr Parker said. 'I saw elderly people floating. I saw one body that really got to me, a child, floating, and it just made me sick.'

Memories of the darkened, storm-battered Superdome were fresh on the minds of evacuees. This time around, the mood was lighter, and evacuees were given water, cookies and misting tents to stay cool while they waited.

Elderly Jeanette Cobbins rode out Katrina in the Superdome but on Saturday was waiting to board a train out of town with 15 relatives in tow.

'We didn't take Katrina seriously,' she said. 'Now we know we've got to get out of town.'

Elsewhere in the city, hospitals prepared to evacuate sickly newborn babies from the intensive care unit.

Katrina was a monstrous Category 5 hurricane on the five-step Saffir-Simpson scale in the Gulf of Mexico before hitting the coast near New Orleans as a Category 3 on Aug 29, 2005, with wind speeds up to 209 kmh.

Its massive storm surge broke through protective levees and flooded 80 per cent of the city. New Orleans degenerated into chaos as stranded storm victims waited days for rescue.

About 1,500 people were killed on the US Gulf Coast and US$80 billion (S$113 billion) in damages made Katrina the costliest US natural disaster. In all, 11.5 million people are in the path of the storm, according to the US Census Bureau.

The US National Hurricane Center said Gustav revved up as it crossed the warm Caribbean and was a Category 4 storm on the five-stage Saffir-Simpson scale of hurricane intensity. It could strengthen into a potentially catastrophic Category 5 storm near Cuba, and hit the US Gulf Coast as a Category 4 storm, the Miami-based forecasters said.

The storm's current projected track takes it into low-lying Terrebonne Parish southwest of New Orleans, one of the least-protected areas on the Louisiana coast.

Mr Windell Curole, manager of neighbouring South Lafourche Levee District, said: 'If it's close to us, our levee system wasn't designed for that kind of storm. There's a tremendous risk.' -- REUTERS