With most of its transportation machinery shut down, the Eastern Seaboard spent the day nervously watching the storm's march across a swath of the nation inhabited by 65 million people. The hurricane had an enormous wingspan - 500 miles, its outer reaches stretching from the Carolinas to Cape Cod - and packed wind gusts of 115 mph.
At least 1.5 million homes and businesses were without power. While it was too early to assess the full threat, Irene was blamed for six deaths.
The hurricane stirred up 7-foot waves, and forecasters warned of storm-surge danger on the coasts of Virginia and Delaware, along the Jersey Shore and in New York Harbor and Long Island Sound. In the Northeast, drenched by rain this summer, the ground is already saturated, raising the risk of flooding.
Irene made its official landfall just after first light near Cape Lookout, N.C., at the southern end of the Outer Banks, the ribbon of land that bows out into the Atlantic Ocean. Shorefront hotels and houses were lashed with waves. Two piers were destroyed, and at least one hospital was forced to run on generator power.
"Things are banging against the house," Leon Reasor said as he rode out the storm in the town of Buxton. "I hope it doesn't get worse, but I know it will. I just hate hurricanes."
By late evening, the storm had sustained winds of 80 mph, down from 100 mph on Friday. That made it a Category 1, the least threatening on a 1-to-5 scale, and barely stronger than a tropical storm. Its center passed North Carolina and was moving along the coast of Virginia. It also was picking up speed, moving at 16 mph.
After the Outer Banks, the storm strafed Virginia with rain and strong wind. Hurricane force winds covered the Hampton Roads region, which is thick with inlets and rivers and floods easily, and chugged north toward Chesapeake Bay.
Maryland transportation officials closed the Chesapeake Bay bridge when wind gusts reached 82 mph. The bridge connects the capital of Annapolis and the rest of Maryland to the Eastern Shore. A tornado touched down in Sussex County in Delaware, damaging at least 15 homes.
Shaped like a massive inverted comma, the storm had a thick northern flank that covered all of Delaware, almost all of Maryland and the eastern half of Virginia.
The deaths included two children, an 11-year-old boy in Virginia killed when a tree crashed through his roof and a North Carolina child who died in a crash at an intersection where traffic lights were out.
In addition, a North Carolina man was killed by a flying tree limb, a passenger died when a tree fell on in a car in Virginia, and a surfer in Florida was killed in heavy waves.
It was the first hurricane to make landfall in the continental United States since 2008, and came almost six years to the day after Katrina ravaged New Orleans. Experts guessed that no other hurricane in American history had threatened as many people.
At least 2.3 million were under orders to move to somewhere safer, although it was unclear how many obeyed or, in some cases, how they could.
Defense Secretary Leon Panetta told 6,500 troops from all branches of the military to get ready to pitch in on relief work, and President Barack Obama visited the Federal Emergency Management Agency's command center in Washington and offered moral support.
"It's going to be a long 72 hours," he said, "and obviously a lot of families are going to be affected."
In New York, authorities began the herculean job of bringing the city to a halt. The subway began shutting down at noon, the first time the system was closed because of a natural disaster. It was expected to take as long as eight hours for all the trains to complete their runs and be taken out of service.
On Wall Street, sandbags were placed around subway grates near the East River because of fear of flooding. Tarps were placed over other grates. Construction stopped throughout the city, and workers at the site of the World Trade Center dismantled a crane and secured equipment.
While there were plenty of cabs on the street, the city was far quieter than on an average Saturday. In some of the busiest parts of Manhattan, it was possible to cross a major avenue without looking, and the waters of New York Harbor, which might normally be churning from boat traffic, were quiet before the storm.
The biggest utility, Consolidated Edison, considered cutting off power to 6,500 customers in lower Manhattan because it would make the eventual repairs easier. Mayor Michael Bloomberg also warned New Yorkers that elevators in public housing would be shut down, and elevators in some high-rises would quit working so people don't get trapped if the power goes out.
"The time to leave is right now," Bloomberg said at an outdoor news conference at Coney Island, his shirt soaked from rain.
A day earlier, the city ordered evacuations for low-lying areas, including Battery Park City at the southern edge of Manhattan, Coney Island with its famous amusement park and the beachfront Rockaways in Queens.
The five main New York-area airports - La Guardia, John F. Kennedy and Newark, plus two smaller ones - waved in their last arriving flights around noon. The Giants and Jets postponed their preseason NFL game, the Mets postponed two baseball games, and Broadway theaters were dark.
New York has seen only a handful of hurricanes in the past 200 years. The Northeast is much more used to snowstorms - including the blizzard last December, when Bloomberg was criticized for a slow response.
For all the concern, there were early signs that the storm might not be as bad as feared. Some forecasts had it making landfall as a Category 3 storm and perhaps reaching New York as a Category 2.
"Isabel got 10 inches from coming in the house, and this one ain't no Isabel," said Chuck Owen of Poquoson, Va., who has never abandoned his house to heed an evacuation order. He was referring to Hurricane Isabel, which chugged through in 2003.
Still, Owen put his pickup truck on a small pyramid of cinder blocks to protect it from the storm tide, which had already begun surging through the saltwater marshes that stand between Poquoson and Chesapeake Bay.
Airlines said 9,000 flights were canceled, including 3,000 on Saturday. Airlines declined to say how many passengers would be affected, but it could easily be millions because so many flights make connections on the East Coast. There were more than 10,000 cancellations during the blizzard last winter.
American Airlines spokeswoman Andrea Huguely said it was not clear when flights would resume out of New York.
"The one thing about a hurricane is that you can prepare for it and you just have to adapt your plan based on how the storm travels," she said. "It's basically an educated guessing game."
Greyhound suspended bus service between Richmond, Va., and Boston. Amtrak canceled trains in the Northeast for Sunday.
The power losses covered at least 1.5 million homes and businesses and were heavily concentrated in Virginia and North Carolina. Dominion Resources reported almost 800,000 customers without power in Virginia. In North Carolina, about 600,000 customers had no power with many of the outages in Wilmington and Wrightsville Beach, N.C. Nearly 55,000 homes and businesses in New Jersey are without power.
Irene roared across the Caribbean earlier this week, offering a devastating preview for the United States: power outages, dangerous floods and high winds that caused millions of dollars in damage.