Hurricane Dorian came to a catastrophic day-long halt over the north-west Bahamas, flooding the islands of Abaco and Grand Bahama with walls of water
At least five people died and 21 injured people were airlifted to the capital by the US Coast Guard, Bahamas officials said.
"We are in the midst of an historic tragedy," prime minister Hubert Minnis said. "The devastation is unprecedented and extensive."
Winds and rain continued to pound the north-west islands late on Monday night into early Tuesday, sending people fleeing the floodwaters from one shelter to another.
"This is unprecedented," said Jeff Masters, meteorology director at Weather Underground. "We've never had a Category 5 stall for so long in the Atlantic hurricane record."
Hundreds of thousands of people in Florida, Georgia and South Carolina were ordered to evacuate before the storm rolls up the Eastern Seaboard, bringing the possibility of life-threatening storm-surge flooding even if the storm's heart stays offshore, as forecast.
Several large airports announced closures and many flights were cancelled for Monday and Tuesday.
The US Coast Guard airlifted at least 21 people injured on Abaco Island, which Dorian hit on Sunday with sustained winds of 185mph and gusts up to 220mph, a strength matched only by the Labour Day hurricane of 1935, before storms were named.
Scientists say climate change generally has been fuelling more powerful and wetter storms and the only recorded storm more powerful than Dorian was Hurricane Allen in 1980, with 190mph winds, though it did not make landfall at that strength.
Abaco and Grand Bahama, neither much more than 40 feet (12 metres) above sea level at their highest points, are home to some 70,000 people.
Bahamian officials said they received a "tremendous" number of calls from people in flooded homes.
One radio station said it received more than 2,000 distress messages, including reports of a five-month-old baby stranded on a roof and a woman with six grandchildren who cut a hole in a roof to escape rising floodwaters. At least two designated storm shelters flooded.
Dorian killed one person in Puerto Rico, at the start of its path through the Caribbean.
Mr Minnis said many homes and buildings were severely damaged or destroyed, but it was too early to say how much the rebuilding effort would cost. Choppy brown floodwaters reached roofs and the top of palm trees on Monday.
Parliament member Iram Lewis told The Associated Press his greatest fear was that waters would keep rising overnight and that stranded people would lose contact with officials as mobile phone batteries died.
"It is scary," he said, adding that Grand Bahama's airport was six feet (almost two metres) underwater and that people were moving shelters as floodwaters kept surging.
The US National Hurricane Centre said Dorian was expected to start moving slowly to the west-northwest overnight while continuing to pound Grand Bahama Island into the morning.
The Centre said the track would carry the storm "dangerously close to the Florida east coast late on Tuesday through Wednesday evening and then move dangerously close to the Georgia and South Carolina coasts on Wednesday night and Thursday".
While it was expected to stay offshore, meteorologist Daniel Brown cautioned that "only a small deviation" could draw the storm's dangerous core towards land.
By midnight local time (5am BST), the storm's top sustained winds had dipped to 130mph but it remained almost stationary. It was centred 25 miles north-east of Freeport.
A mandatory evacuation of entire South Carolina coast took effect on Monday covering about 830,000 people.
Georgia Governor Brian Kemp ordered mandatory evacuations for that state's Atlantic coast, also starting at noon on Monday.
Authorities in Florida also ordered some mandatory evacuations.
FlightAware.com reported that that airlines had cancelled 1,361 flights within, into or out of the US by Monday afternoon - vastly above an average day - with Fort Lauderdale International the most affected, and airlines had already cancelled 1,057 flights for Tuesday, many involving Orlando, Fort Lauderdale and Miami airports.
A hurricane watch was in effect for Florida's East Coast from Deerfield Beach north to South Santee River in South Carolina. A storm surge watch was extended northward to South Santee River in South Carolina. Lake Okeechobee was under a tropical storm watch.