Hurricane Katrina - a fierce mass of winds swirling at 155mph (250km/h) - made landfall in the US state of Louisiana early Monday local time.
Large parts of the city of New Orleans are under water.
BBC correspondents report from the city and from Baton Rouge, to the north-east, on the storm and its aftermath.
Alastair Leithead : New Orleans, Louisiana : TUESDAY 0649 GMT
As the high winds began to ease, the people who had decided to stay in New Orleans and checked into hotels for safety emerged to see just what Hurricane Katrina had done.
This historic area around the French quarter and the few blocks of downtown fared quite well. There was a lot of debris, windows were blown in, metal signs ripped from brickwork, but no flooding. People here began to think that maybe New Orleans had got away with it.
But those who had remained at home had a different story to tell. Downtown is now an island of slightly higher ground in an area which is mostly below sea level and was flooded by torrential rain which had nowhere to go and breached storm dams.
People only had a few shell-shocked hours of daylight to assess the damage and many remain overnight in their temporary homes and shelters. But there's no power, and communications are down.
Adam Brookes : Baton Rouge, Louisiana : 0558 GMT
From far-flung towns along America's Gulf coast, information is trickling in slowly in the wake of Hurricane Katrina.
From Biloxi, Mississippi, a small resort town, are coming reports of very severe storm damage. One report speaks of large parts of the town destroyed. A local official has been quoted as saying more than 50 people have died there.
In an internet posting, Biloxi's mayor says the town is effectively cut off, with roads impassable and telephone lines cut. The mayor said the conditions were making rescue operations difficult.
The true accounting of the damage caused by Hurricane Katrina is now beginning. But it will be many hours and days before the extent of the damage in outlying areas is known.
Adam Brookes : Baton Rouge: 0445 GMT
Hurricane Katrina ploughed onto the US mainland, the eye of the storm narrowly missing the city of New Orleans.
The hurricane weakened as it made its way inland and by evening, had been downgraded to a tropical storm.
New Orleans suffered serious damage, though not as severe as earlier feared. The city's famous old French Quarter has survived mostly intact. But the low-lying suburbs have seen serious flooding.
About 200 people stranded on their rooftops by rising waters were rescued by emergency services. Those operations were continuing into the night, with police boats shining searchlights down flooded streets.
Alastair Leithead : New Orleans, Louisiana : MONDAY 1520 GMT
I've come out to the ground floor of the hotel I'm staying in, and I can peer round this corner and see the wind at the top of the hotel.
The first thing I can see is palm trees, two palm trees that are really leaning in a direction they shouldn't be leaning, and a huge amount of broken glass.
I can't get too close in case the glass blows up into my face. Obviously the wind had swirled round. A couple of windows seem to have blown out. The street is covered in debris.
Broken signs, pieces of metal, the newspaper stands they're all upturned. The power cables are all down.
I think I've seen enough to get an idea of just how bad this is. If I was to stand on the other side of the road I would immediately be swept away.
The wind is over 100mph (160kph). Police we were talking to suggested it may even be as much as 120mph (193kph). It lifted a car completely off its back wheels and moved it further along the road.
So that's the scene in this little part of the French quarter in New Orleans.
Alastair Leithead : New Orleans, Louisiana : 1230 GMT
Power has gone down in this hotel where I am staying but at the moment, the wind doesn't seem to be doing that much damage to buildings and houses.
It is now difficult for us here to monitor how far away the eye of the storm is. Exactly when and where it will hit remains to be seen.
Because visibility is so low it is difficult to say quite how the dams are doing in keeping water out.
When I last went outside palm trees along the boulevard looked like they were going to topple over quite soon.
Things are fine when it doesn't wobble the windows. But when they start moving it gets frightening because I don't know how much these windows can take.
Alastair Leithead : New Orleans, Louisiana : 1200 GMT
Gradually as time has ticked by, the winds have increased in strength. The rain storms became walls of water running down the sides of skyscrapers.
The crack of power lines shorting out punctuated the sound of air rushing past buildings at speeds of up to 155mph (250kph).
Anything not tied down was being blown across the street. And still the winds intensify. The eye of the storm has yet to reach New Orleans. It will get worse before it gets better.
The future of the city lies in the hands of its levees, the dams built to help to hold back the swell of hurricanes but hardly expected to cope with one of the worst storms this country has ever experienced.
Power cuts came as dawn broke. The waiting is over. People can only hide themselves until Hurricane Katrina moves away.
Alastair Leithead : New Orleans, Louisiana : 1110 GMT
This is the worst it has got, to be honest with you. I have just been outside to have a little look at what has been going on.
Basically, of the line of palm trees outside, one of them is already down. There is a huge palm tree lying across the road.
Rivers are running along the streets. There is a crackling noise all around - it must be electricity cables shorting out, as well as the generators. And the water is coming down intensively.
The winds are really picking up now. They are not at hurricane force yet but certainly they are strengthening all the time and the rain is coming down.
I am sheltering in a high rise building like a lot of other residents. I hope it is safe.
Adam Brookes : Baton Rouge, Louisiana : 0710 GMT
We are expecting the storm to hit in about seven hours' time and the full force will hit us about three hours after that.
The forecasters are calling this a category five, they are saying that it is showing signs of weakening ever so slightly so it may become a strong category four.
But on the ground that is academic - for people in the path it is going to be a very, very strong storm indeed.
They are talking about 15 inches (38cm) of rainfall and a storm surge that could raise the level of rivers and lakes in this area by several metres.
Alastair Leithead : New Orleans, Louisiana : 0525 GMT
The streets are quiet. Those who want to leave New Orleans have left and for those who have stayed behind, there's a tense apprehension as Hurricane Katrina gradually moves towards land.
People have left their homes and moved into high-rise hotels, hoping they'll be safer there.
Emergency shelters have been filling up, the biggest being the main sports stadium which is the nearest the city has to high ground.
Alastair Leithead : New Orleans, Louisiana : 0344 GMT
New Orleans is pretty much right on the line of the approach of the storm. They eye of the hurricane will probably pass straight over, or very close, to the city itself.
There has just been a huge flash of lightening - the storm is getting much closer, the wind is picking up. There's the thunder, the first rumbles of thunder that I've heard this evening.
When the water comes, people are saying maybe as much as two or three metres in some parts of the city, it will cause a huge amount of damage.
I'm not sure whether the people are aware of what is going to hit them, they have certainly not experienced anything like this in this part of the world.