Hurricane Florence is "wreaking havoc" along the US coast and could wipe out entire communities, North Carolina's governor has warned.
Buildings were torn apart by 90mph winds and hundreds of people were trapped by high water as the Category 1 storm settled in for what could be an extraordinarily destructive drenching.
Governor Roy Cooper called the rainfall an event that comes along only once every 1,000 years.
"Hurricane Florence is powerful, slow and relentless," he said. "It's an uninvited brute who doesn't want to leave."
The centre of the hurricane made landfall near Wrightsville, North Carolina, at 7.15am local time.
Its storm surge and the prospect of 1-3ft of rain were considered a bigger threat than its winds, which had dropped off from an alarming 140mph earlier in the week.
Forecasters said catastrophic freshwater flooding is expected well inland over the next few days as Florence crawls westward across the Carolinas all weekend.
The area is expected to get about as much rain in three days as Hurricanes Dennis and Floyd dropped in two weeks in 1999.
Preparing for the worst, about 9,700 National Guard troops and civilians were deployed with high-water vehicles, helicopters and boats that could be used to pluck people from the floodwaters.
For people living inland in the Carolinas, the moment of maximum peril from flash flooding could arrive days later, because it takes time for rainwater to drain into rivers.
Authorities warned too of the threat of mudslides and the risk of environmental havoc from floodwaters washing over industrial waste sites and pig farms.
Florence was seen as a major test for the Federal Emergency Management Agency, which was heavily criticised as slow and unprepared last year for Hurricane Maria in Puerto Rico, where the storm was blamed for nearly 3,000 deaths.
The National Hurricane Centre said Florence will eventually make a right turn to the north east over the southern Appalachians, moving into the mid-Atlantic states and New England as a tropical depression by the middle of next week.
Meteorologist Ryan Maue, of weathermodels.com calculated, that 34 million people in the US could get at least 3in of rain from Florence, with more than 5.7 million people probably receiving at least a foot.
Florence is expected to dump about 18 trillion gallons of rain over a week along its entire path, enough water to fill more than 65,000 Empire State Buildings, Mr Maue calculated.
On Friday, coastal streets in the Carolinas flowed with frothy ocean water, and pieces of torn-apart buildings flew through the air. The few cars out on a main street in Wilmington had to swerve to avoid fallen trees, metal debris and power lines.
The Wilmington airport had a wind gust clocked at 105mph, the highest since Hurricane Helene in 1958, the weather service said.
Hurricane Florence, growing in size despite its weakening winds, churned ever closer to the U.S. East Coast on Wednesday as evacuations expanded south from the Carolinas into Georgia to counter the threat of deadly high seas and calamitous floods.
Hurricane Florence is expected to strengthen into a near-Category 5 storm as it approaches the US where 1.7 million people have been warned to get out of the way.