Thousands of troops, rescue workers and volunteers have been deployed in Louisiana and neighbouring Gulf of Mexico states following what is being called the biggest evacuation in US history.
Gustav’s arrival comes days after the third anniversary of Hurricane Katrina, which devastated New Orleans in 2005 leaving more than 1,000 dead and causing $30bn (£16.5bn) in damage. It was a disaster from which the city has not yet recovered.
Hundreds of police and national guardsmen are patrolling the city’s empty streets to deter looters and take action if a major rescue operation is required.
"The outer edge of the storm is already over the Mississippi Delta and going in toward New Orleans now," Patricia Wallace of the US National Hurricane Centre told the AFP news agency.
The eye of the storm is expected to make landfall around midday (6pm GMT). The heart of the hurricane was 275km (170 miles) south-east of New Orleans at midnight, moving towards the city at 26kph.
Storm force winds from Gustav extended up to 350km from the eye, according to the hurricane centre. Gustav is still classed as a category three hurricane, with winds of up to 185kph.
Weather forecasters said that despite Gustav losing power after hitting Cuba on Saturday, it could gain strength as it moved across the Gulf of Mexico.
The storm will test three years of planning and rebuilding that followed Katrina’s devastation. Gustav has already killed at least 94 people in the Caribbean.
Determined to avoid a repeat of the failings that led to more than 1,600 deaths during Katrina, US officials have moved beyond merely insisting tourists and residents leave south Louisiana. They threatened to jail looters, loaded thousands of people on to buses and warned that anyone who remained behind would not be rescued.
Ray Nagin, the mayor, warned residents they were facing the "storm of the century", and imposed a curfew from sunset last night. "This is the real deal, not a test," he said yesterday.
The storm forced the Republican party to cancel most of the programme for the first day of its four-day convention, which had been scheduled to begin today in Minneapolis-St Paul. President George Bush and Vice-President Dick Cheney cancelled plans to speak at the convention tonight.
Bobby Jindal, Louisiana’s governor, warned that storm surges could mean the waters would overflow levees. He ordered tens of thousands of national guard members to deploy.
The main tourist district of New Orleans, the French Quarter, was eerily subdued yesterday. Bars, hotels and souvenir shops had locked their doors. Lights were left on and music was played in some. Although the district is on high ground and escaped flooding last time, a few buildings had panels of wood hammered over the doors and windows as a precaution. One stood out with bright pink-sprayed letters: "Don’t even think about it, Gustav."