Hurricane Katrina - a fierce mass of winds swirling at 155mph (250km/h) - hit the US Gulf coast on Monday, causing widespread devastation.
President George W Bush has described it as "one of the worst natural disasters" the US has seen.
BBC correspondents report from across Louisiana and Mississippi as authorities struggle to restore order.
Jamie Coomarasamy: Baton Rouge, LA : 06:45 GMT
With every day the level of desperation here is rising. Thousands of people are still trapped inside New Orleans in a city where the authorities appear overwhelmed by the scale of the disaster.
Like many of the people living in unsanitary and dangerous conditions in the city, local officials are now pointing fingers, in their case at the federal government.
The head of New Orleans emergency operations described the relief effort as a national disgrace. He asked why America can send massive amounts of aid to tsunami victims but can't bail out his city.
Meanwhile, thousands of national guardsmen are due to arrive in Louisiana from across the United States to try to stem a tide of lawlessness which threatens to leave scars on the community long after the floodwaters have receded.
Daniel Lak: New Orleans, LA : 04:45 GMT
The authorities in the state of Mississippi say they won't have extra personnel to make special security arrangements for the visit of president Bush to the storm-ravaged Gulf Coast on Friday.
Secret service and other federal agencies are expected to compensate for that. All police and law enforcement officials in this area are needed for flood relief operations in the wake of Hurricane Katrina.
President Bush will see the massive floating casinos that have been hurled up on shore, thousands of smashed homes and downed power-lines, and perhaps gatherings of homeless people, for there are many of them here.
The president's handling of the crisis so far has been criticised by some local people in this area, who've told reporters they thought he should have done more - and sooner.
Politics aside, the president's visit should help speed up the rescue operation, which many here feel has been too slow to get started.
Peter Marshall: New Orleans, LA : 21:44 GMT
I've spent the day touring New Orleans now under 20ft of water in some places. Around the city's convention centre the scene is ugly, there has been violence and shots have been fired.
There is no food, there are no toilets.
The need to seek protection is instinctive, people here are afraid, they gather in huddles near the police.
Canal Street is now itself a canal. New Orleans is utterly and perhaps forever changed.
David Willis: Biloxi, MS : 21:10 GMT
The aftermath is posing a major challenge for rescue teams charged with checking every building in this costal town. The force of the hurricane was unbelievable. Sniffer dogs have been brought in to help with the search, as well as manpower from every state in the union.
Hotel guests use bed sheets on their balconies to beg for help. Bulldozers have started clearing the streets after rats were seen gathering amongst the rubble.
Officials here are estimating that nearly 1,000 have died in this community.
Matt Frei: New Orleans, LA : 21:03 GMT
I drove into the city just a few hours ago and it was one of the scarier journeys I have made.
It looks like the set of a Hollywood disaster movie.
The battle between nature and man is almost over, but the battle between man and man is just beginning. The scene here is more Africa than America.
From the air the destruction is humbling, luxury yachts thrown about like toys. Looking closer there is stagnant, dirty water, and then there are the survivors, mostly poor, black and angry. You see dead bodies here as you wander around the streets.
Adam Brookes: New Orleans, LA : 20:35 GMT
On the approaches to New Orleans a trail of people - those who've decided to walk out of the city - line the highways.
In the centre of the city thousands have gathered outside the stadium waiting for a seat in the long convoy of buses and evacuation. Most waited without any food or water.
A group of elderly residents of a care home in wheelchairs sat bewildered in the ninety degree heat and rain. The evacuation appears agonisingly slow and it's unclear why, after three days, so many people seem to be without relief supplies.
One man, fury and exhaustion etched on his face, shouted at me: "This is America, why are we in this situation?"
Daniel Lak, Gulf Port, MS : 18:50 GMT
This already poor part of America has been utterly devastated by Hurricane Katrina and the authorities are struggling to cope.
The top concern is public health, with most drinking water contaminated. A mandatory order to boil all tap water has been issued. Looters are being warned to stay away from damaged shops by the police and many shop owners are guarding their premises with guns.
It will take years to get this area even remotely back to normal.
Jonathan Beale, Washington D.C. : 16:42 GMT
We have been hearing about the relief efforts at a White House press briefing. We know President Bush has asked for extra finances from Congress for aid, he wants to make sure there is enough funds for the clean up, which is going to take a long time.
President Bush is under pressure for his handling of this, but so far most of the criticism has been in the media. The New York Times said that Bush's speech yesterday was one of the worst speeches of his life.
There is criticism from the people struggling on the ground too, but so far there has been no vocal criticism from other politicians.
The hurricane having gone, the political storm will now brew.
Jamie Coomarasamy, Louisiana : 16:21 GMT
Evacuees from New Orleans continue to pour into Louisiana's state capital, swelling this city's numbers and stretching its resources.
I am at the department for homeland security in Louisiana and they are clearly struggling here. They are very aware of the problems with lawlessness. One official told me that it was a difficult task for the law enforcement people but that they would be successful in getting everyone out.
Some of the shelters here have over 8,000 people. They are turning up with nothing more than the clothes they are wearing, wandering about aimlessly, looking for loved ones.
Alastair Leithead, New Orleans, MS : 16:08 GMT
It is pretty lawless here now on the streets of New Orleans. It has been deteriorating over the last 24 hours including a shooting at the Dome where people are staying.
I can hear a rescue helicopter overhead now, people are desperate, they have no way of leaving.
There is no evidence of co-ordination, no details, no buses - people have no idea how they can get out of here.
There is real anger here too that things are not being done quickly here. There are groups of one and two thousand people together in different places but there is no aid being given to them.
There have been shoot outs between police and armed looters. It is difficult to define looting, is it people just trying to get food and clothes or real looters trying to steal what they can get.
Daniel Lak, Gulf Port, MS : 16:00 GMT
There's not much left of the shops, businesses, hotels and casinos that used to line the beach front here and in neighbouring Boloxi but there have been reports of people looking through the rubble for cash or luxury goods.
Rental car agencies have posted guards and the police patrol frequently, especially in the pitch black of night.
Emergency management officials are trying to co-ordinate a rescue effort on the huge scale necessary to meet the needs of more than 100,000 needy people, perhaps more, many of whom have lost their homes and all their possessions.
The death rate could also climb a searches continue for survivors in areas still cut off three days on.
Alastair Leithead, New Orleans, LA : 15:05 GMT
A few moments ago a man came up to me and asked if I was a journalist. He said: "Can you help me? I need to save the lives of 60 women."
Twelve blocks away there are 60 people in an old people's home and 30 staff who have been looking after them since the hurricane struck. He said they had enough water to last 12 hours, two of the women had died already and three more will probably die today.
"Who can I ask?" he said. I said I didn't know and he wandered off. The policemen wouldn't talk to him so he's gone off to try to find someone who might help.
That's just one isolated case and there are many more like that across this city. It is a desperate situation. There is a real feeling here that not enough is being done and certainly not fast enough.
Jamie Coomarasamy, Baton Rouge, LA : 15:00 GMT
President Bush has said there will be zero tolerance for people who break the law on the Gulf Coast.
He is arriving to see the devastation for himself on Friday and is expected to call on his two predecessors in the White House - President Clinton and President Bush Senior, to help raise funds.
Alastair Leithead, New Orleans, LA : 13:30 GMT
We've just heard that the evacuation operation that has been taking place at the Dome, the football stadium where up to 20,000 people have been staying, has been suspended because shots were fired within the dome.
Apparently shots were also fired at a rescue helicopter that was passing overhead.
There really is an edgy feeling about the city, it is no longer comfortable and it doesn't seem as if the emergency services are dealing with this very well.
Alastair Leithead, New Orleans, LA : 12:10 GMT
This whole city is going to be evacuated - everybody is going to leave and they don't know when they are going to be allowed to come back.
There's a real sense of desperation amongst those people who've waded and swum their way from their flooded houses to the only part of dry land in downtown to find that there's no water, no food and no plan to get them out.
People are being evacuated but it's a very slow process and the authorities, it appears, are struggling to cope.
Jonathan Beale, Washington, DC : 12:05 GMT
There's concern, first of all, about the national guard. We've seen those images - and so have the American public - of the widespread looting in the region. Questions are being asked - well, where are the forces of law and order?
And then there are questions about the cost of the clean-up and the rescue.
America is spending $200bn on Iraq and people want to know there's plenty more money to help their own people. President Bush reassures people America can fight its war on terror and do this clean up but he's appealing for patience too.
Alastair Leithead, New Orleans, LA : 11:45 GMT
There's a peculiar feeling of danger and foreboding here in New Orleans, as those people left marooned in the city are becoming increasingly desperate.
Armed gangs have moved into some hotels and looting has reached an alarming level. There have been shootouts between gangs and criminals and there are reports that martial law has been imposed across parts of the city.
The mayor has pulled the majority of his police force away from the search and rescue effort to provide backup for those fighting crime.
Adam Brookes: New Orleans, LA : 11:30 GMT
The most striking thing is the sheer level of resources and power of the American emergency machine when it gets going. People knew this storm was coming - they were able to evacuate and prepare to some degree.
If you look around the edges of New Orleans - we've been down there taking a look - you can see the colossal resources being brought in pretty quickly. Columns of hundreds and hundreds of speedboats drawn by pickups arrived practically instantaneously. The American military has got helicopters lifting off and touching down by the moment.
America can deal with these things effectively but equally the American public has much higher expectations of its government.
We've just been hearing President Bush being questioned for the first time by journalists about why the relief effort is perceived to be quite slow and I think the Bush administration may be in some trouble here.