Hurricane Isaac has weakened to a tropical storm, pummelling New Orleans, but causing most damage to less-protected areas outside the city.
Isaac is soaking the US Gulf Coast exactly seven years after the devastating and much stronger Hurricane Katrina.
An overnight curfew has been declared by the mayor of New Orleans.
Its multi-billion-dollar barriers have stood firm, but sea water has breached a levee in a town south of the city.
At 22:00 (03:00 GMT Thursday), the storm was about 15 miles (25km) south of the city of Baton Rouge, and 70 miles (110km) west of New Orleans, according to the US National Hurricane Center (NHC).
The NHC warned that the rain threat from Isaac will spread inland over the next few days.
The ponderous storm, moving at about 6 mph (10km/h), could take most of the day to travel as far as Baton Rouge, a town 70 miles (110km) to the north-west of New Orleans.
The storm was downgraded from a hurricane to a tropical storm on Wednesday afternoon.
But Louisiana Governor Bobby Jindal told reporters on Wednesday that the state would be dealing with Isaac "through early Friday morning".
The residents of Plaquemines Parish, where Isaac first touched down, about 65 miles (105km) south of New Orleans, have bore the brunt of the storm so far.
Authorities have said they plan to cut a hole in one of Plaquemines' levees, which are not part of, or as high as, the upgraded system around New Orleans, in order to relieve pressure from flooding.
Dozens of people in the area have been rescued.
Caitlin Campbell, a spokeswoman for Plaquemines Parish water was running over an 18-mile (29-km) stretch of the 8ft-high levee and some homes were flooded.
After hitting Plaquemines, the then-category one hurricane headed back out to sea, before making a second landfall further west at Port Fourchon at about 02:00 (07:00 GMT) local time.
There were reports of serious flooding and people stranded by rising waters in the Braithwaite area, just across the Mississippi river from metropolitan New Orleans.
'Worse than Katrina'
Plaquemines Parish President Billy Nungesser said the damage wrought by Isaac was worse than Katrina.
"I don't know who's calling this a category 1, but this is no category 1," Mr Nungesser said. "My house has more damage than it did during Katrina."
Strong winds were hampering full-scale air and water rescue efforts in the area, but the BBC's Alastair Leithead reports local residents were trying to make rescues with small boats. The National Guard was also set to launch a rescue effort.
Storm surges are still a concern, with peaks of up to 12ft (3.7m) forecast in parts of Mississippi and south-eastern Louisiana.
Rainfall of up to 1ft 8in is expected across wide areas, with a high chance of isolated tornadoes along the coast.
The bowl-shaped city of New Orleans is particularly vulnerable to storms, with the centre of the city the furthest below sea-level.
Many residents of New Orleans have chosen to secure their homes but stay put, saying they were not too concerned by Isaac.
Officials are urging residents who have not evacuated to stay in place until high winds weaken.
President Barack Obama has declared an emergency in Louisiana and Mississippi, allowing federal funds to be released to local authorities.
About 600,000 people were without power across five states, the majority in Louisiana, as of Wednesday afternoon.
Isaac killed at least 24 people as it passed over Haiti and the Dominican Republic earlier this week.
It also caused significant flooding and damage across the Caribbean and forced a day's delay to the start of the Republican party's congress in Tampa, Florida.