Hundreds of thousands of people were fleeing New Orleans yesterday after the city's mayor warned "the storm of the century" was coming and it was "time to be scared".
And President George W Bush said Hurricane Gustav poses a "dangerous" threat to residents of the city still recovering from the devastation of Katrina three years ago.
Roads out of the port were crammed with traffic and authorities have been trying to help evacuate anyone who cannot get themselves out.
The hurricane, set to hit the US Gulf Coast today, has already left a trail of death and destruction in its wake, killing more than 80 people across the Caribbean.
New Orleans mayor Ray Nagin told residents: "You need to be scared; you need to get your butts out of New Orleans now.
"This is the mother of all storms. I am not sure we have seen anything like it."
He said staying behind would be one of the biggest mistakes of their lives.
"Anyone who decides to stay, I'll say it like I said it before Katrina: make sure you have an axe, because you will be carving your way, or busting your way out of your attic to get on your roof with waters that you will be surrounded with in this event."
Later, Mr Nagin imposed a dawn-to-dusk curfew from sunset as Gustav approached.
The hurricane dropped from a Category 4 to a Category 3 storm overnight, but forecasters warned it could gain strength from the Gulf's warm waters before making landfall as early as today.
Mr Bush, severely criticised for his administration's poor handling of the Katrina disaster, urged residents on the US Gulf coast to heed warnings and added: "Know that the American people stand with you and we will face this emergency together."
Speaking at the headquarters of the Federal Emergency Management Agency (Fema), the president said governors and local leaders across the Gulf coast were "taking this storm very seriously and preparing pro-actively".
Gustav was "dangerous" and posed a "serious risk of significant flooding", he said.
"This nation has come to know the strong and resilient spirit of the people of the Gulf Coast.
"They've made it through great challenges in the past and they're going to make it through this one, as well."
President Bush and vice president Dick Cheney cancelled their plans to attend the Republican Party's national convention, which is due to start in St Paul, Minnesota.
Both were due to speak there tonight.
Meanwhile, Mr Nagin said that looting, one of the major problems in the wake of Hurricane Katrina three years ago, would not be tolerated.
"Looters will go directly to jail," he said.
The US authorities' response will be closely scrutinised after what was widely seen as a series of catastrophic failures in the wake of Katrina.
After Katrina hit land on August 29, 2005, three-quarters of the city was flooded after the storm surge breached its protective levees and more than 1,800 people were killed in US coastal areas.
But a Briton who runs a pub in New Orleans said he would defy the mayor's order to evacuate the city.
James, who declined to give his surname, said: "I will probably stay until someone with a rifle and uniform shows up. I am staying here because of what happened to my pub when Katrina rolled in – looting and mindless destruction."
The Crown and Anchor is located on the west bank of the Mississippi River, which is being evacuated.
But James, who left Putney, south-west London, for New Orleans 18 years ago, said a number of Algiers Point residents were planning to ignore the mandatory evacuation order.
The 38-year-old said: "This neighbourhood is like a small village. The families with small children have already left, but I know there are at least 20 of us planning to stay on.
"The general consensus here is the mayor is covering his back after the colossal muck-up over Katrina."
He said the atmosphere in Algiers Point was fairly calm and those staying behind were getting ready to batten down the hatches.
"Everyone seems pretty level-headed and we are preparing appropriately.
According to Foreign Office advice given to Britons yesterday, all hotels in New Orleans are now closed.
The Foreign Office told them: "You should monitor local television and radio for updates on the latest situation and follow the advice of local authorities."
UK travel association ABTA said the number of British tourists in the city would be "fairly small" and that the majority of Britons there were likely to be students living abroad.
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Weather for Yorkshire
Monday 20 May 2013
Temperature: 11 C to 19 C
Wind Speed: 14 mph
Wind direction: North
Temperature: 7 C to 15 C
Wind Speed: 14 mph
Wind direction: North