Evacuations have already begun in Maryland, New Jersey, New York City, North Carolina and Virginia.
Tornado warnings have been issued in North Carolina. The core of the hurricane hit the North Carolina coast Friday night and is expected to pass near or over the coast on Saturday.
States of emergency have been declared in North Carolina, Virginia, Maryland, New Jersey, New York, Connecticut, Rhode Island, New Hampshire, Massachusetts, and Washington, D.C.
President Barack Obama, who spoke from his vacation rental on Martha's Vineyard Friday, warned people in the path of the hurricane to prepare for the worst and take precautions.
"I cannot stress this highly enough. If you are in the projected path of this hurricane, you have to take precautions now," Obama said. He was wrapping up his vacation a day early and now planned to leave Friday, before Irene is expected to pass the area around the capital, the White House said.
The hurricane may be the strongest to strike the East Coast in seven years, and people were already getting out of the way.
Several states have declared a state of emergency ahead of Irene. Hurricane warnings remained in effect from North Carolina to New Jersey. Hurricane watches were in effect even farther north and included Long Island, Martha's Vineyard and Nantucket, Mass.
In addition to widespread wind and water damage, Irene could also push crude oil prices higher if it disrupts refineries in Delaware, New Jersey, Pennsylvania and Virginia, which produce nearly 8 percent of U.S. gasoline and diesel fuel.
In New York, Mayor Michael Bloomberg ordered mandatory evacuations for about 250,000 residents in low-lying coastal areas. The areas include the Battery Park City complex on the southern end of Manhattan; Coney Island, famed for its boardwalk and amusement park; the beachfront community of the Rockaways and other neighborhoods around the city.
The sun was shining outside, but Bloomberg called it "the calm before the storm."
New York's subways and other mass transit will be shut down starting at noon on Saturday.
Shelters were set to open and other preparations continued as the city faced the likelihood of its first hurricane warning since 1985.
Heightened waves began hitting the North Carolina's Outer Banks early Friday. On boarded up windows, residents in Nags Head, North Carolina, boasted of conquering previous storms. Alan Ross was one of many residents preparing for the worst.
"The most important thing that we worry about is projectiles. The wind can throw stuff around, and anything become a projectile," he said.
At the Ace Hardware store, supplies were running low. Charles Johnson was there buying a carpet cleaner.
"Get up when the storm hits, and start sucking water up," Johnson said.
Chino native Stephanie Carmona said she's heading out because of her kids, but she wishes she could stay.
"I'm from California, so I'm used to earthquakes and dry weather, so this stuff is awesome," Carmona said.
IRENE AFFECTS TRAVEL PLANS
Hurricane Irene is expected to impact travel plans. People heading to the East Coast or the Bahamas were warned to double check their flight status ahead of time.
At LAX, passengers scrambled to get aboard the remaining flights to North Carolina, Maryland, Connecticut, New York City and Washington D.C.
The airlines have already scrubbed over 6,000 in anticipation that bad weather will shut down over two dozen East Coast airports. Amtrak has canceled service below Washington D.C.
All flights are expected to be halted by midday on Saturday.
Airlines are waiving rebooking fees for passengers who want to delay their flights. Regular schedules should resume on Tuesday.
The Associated Press contributed to this report.