The monster storm Katrina raged ashore along the Gulf Coast Monday morning, ripping off roofs, shattering windows and causing widespread flooding and vast power outages before heading inland towards the northeast.

New Orleans and the Mississippi cities of Biloxi and Gulfport, as well as dozens of smaller coastal communies, were said to have been hard hit after Katrina came ashore with winds of about 140 mph, making it a category 4 storm. By 11 a.m. EDT, Katrina was category 3 with maximum wind speed of 125 mph, still dangerous.

While it was difficult to assess damage early in the day, there were numerous early reports of buildings collapsing along the coast, roofs blowing apart, windows flying out of office buildings and major flooding.

New Orleans' Superdome, serving as a shelter for about 10,000 people, lost power and was leaking from the roof, parts of which flew off. The roof of a hospital was reported to be blowing off just outside of the city in Chalmette.

Gulfport Fire Chief Pat Sullivan told wire services that his city had suffered "a devastating hit."

Bong Q. Mui, chief of staff at Chalmette Medical Center, located just outside of New Orleans, reported heavy rain and strong winds and reported at 10:30 a.m. EDT that the roof of the hospital was blowing off. He said he had heard that parts of Interstate 90 were under four feet of water and that the roof of the local high school was gone.

The hospital, which has about 50 patients who could not be evacuated, lost electricity and was operating on emergency back-up power.

Some windows were blown out of office buildings in New Orleans even before the brunt of the storm arrived.

The weather service stressed that Katrina, although reduced in power, remained a life-threatening force. "This is still an extremely dangerous and potentially deadly hurricane," said a statement from the National Hurricane Center.

"There will be a tremendous amount of property damage," Louisiana Gov. Kathleen Babineaux Blanco (D) said in an interview on CNN. "We don't know yet. We still have a long way to go. We are watching. We are worried."

The fate of the Superdome underscored that warning. Pieces of metal sheared off, leaving two holes that were visible from the floor, the Associated Press reported. Water dripped in and people were moved away from about five sections of those seats.

The development, around 9 a.m. EDT did not create any visible fear among the thousands who spent the night in the huge arena. Some watched as sheets of metal, flapping visibly, rumbled loudly, the AP said. From the floor, looking up more than 19 stories, it appeared to be openings of about 6 feet long.

General Manager Glenn Menard said he did not know how serious the problem was. "We have no way of getting anyone up there to look," he said.

Gov. Blanco said "nothing has impaired the lives" of people in the Superdome and that there was no structural damage.

Katrina made landfall at 7:10 a.m. EDT at Buras, La., in Southern Plaquemines parish. It was a Category 4, down from last night, with maximum winds of 140 mph to the east of the storm center. The eye was roughly 32 miles in diameter. Hurricane force winds extended 120 miles outward from the eye with tropical storm force winds of 60 and 70 mph extending outward for 230 miles.

Richard Knabb, of the Hurricane Center, said Katrina was headed for the Louisiana-Mississippi border. Katrina's projected route after landfall hugs a stretch of warm Gulf water from which it could suck more energy and intensify, the weather service said.

Power outages began in the New Orleans area starting about 6 a.m. In addition to the Superdome, other large structures -- including schools and hotel ballrooms -- had been transformed into refugee centers, all of them packed. Three thousand people took cover in the Hyatt Hotel ballroom in downtown New Orleans.

Millions of residents of New Orleans and other Gulf communities evacuated, although hundreds of thousands remained, preparing to take cover as they could.

New Orleans in particular worried about the storm surge -- huge waves whipped up by the enormous force of the hurricane winds -- which could very well overcome the levees of the city.

Oil companies shut down production from many of the offshore platforms that provide a quarter of U.S. oil and gas production. U.S. oil futures jumped nearly $5 a barrel in opening trade to touch a peak of $70.80. The rise in oil prices fed through to other financial markets, hurting stocks and the dollar on fears that economic growth might be curtailed but boosting safe havens such as government bonds and gold.

While Katrina was technically a Category 4 storm Monday morning at 155 mph, officials cautioned that there had not been a significant change in the storm's intensity.

"Just because Katrina is no longer a Category 5 hurricane does not mean that extensive damage and storm surge flooding will not occur," said a statement from the Hurricane Center.

New Orleans Mayor Ray Nagin ordered the mandatory evacuation of the city's 485,000 residents. Officials acknowledged that tens of thousands of residents and tourists would be unable to leave. With the airport closed, the city organized buses to transport those left behind to 10 emergency shelters and encouraged people to bring supplies and food for a three- to five-day stay. Three nursing home patients died during the evacuation, according to an Associated Press report.

"This is a once-in-a-lifetime event," Nagin said at a televised news conference Sunday evening. "The city of New Orleans has never seen a hurricane of this magnitude hit it directly. . . . We are facing a storm that most of us have long feared."

Officials said Katrina's wrath could easily surpass the devastation caused in 1965 by Hurricane Betsy, the most punishing storm to hit southeastern Louisiana. That storm killed 75 and caused $7 billion in damage when southern Louisiana was less populated and less exposed.

The American Red Cross said it had set up 35 kitchens in Louisiana, Mississippi and Alabama that could serve 700,000 meals a day. Meanwhile, President Bush declared a state of emergency in Louisiana and Mississippi, making it easier for federal agencies to coordinate relief efforts with states and localities.

New Orleans is especially vulnerable to a hurricane's fury because the city sits six to eight feet lower than the surrounding waters of the Mississippi River, Lake Pontchartrain and the Gulf of Mexico. And it is sinking lower every year.

The levees that harnessed the Mississippi and helped make New Orleans one of the world's busiest ports and a thriving center of the oil and gas industry also have prevented the river from spreading sediment around its delta. As a result, southern Louisiana is slowly sinking into the encroaching Gulf, losing about 24 square miles of coastal marshes and barrier islands every year. Those marshes and islands used to help slow storms as they approached New Orleans; computer simulations have suggested that their loss will increase storm surges and waves by several feet.

"If you can picture sort of a soup bowl, the city is located in the middle," said Gregory W. Stone, director of the Coastal Studies Institute at Louisiana State University in Baton Rouge. "And once the levees -- sort of the perimeter of the bowl -- are breached or overtopped, then that water gets in there and just can't get out . . . . It's like filling a bucket up with water. This is probably the worst-case scenario that we've all been very, very worried about for quite some time."

Nagin said Katrina's predicted storm surge would probably overwhelm New Orleans's levees. And the city's pumps -- capable of removing only an inch of water every hour under normal conditions -- require electricity, often the first casualty in any hurricane. If Katrina does slam into New Orleans, experts say the city could be underwater for weeks or months, creating a toxic soup of chemicals, rodents, poisons and snakes.

Katrina formed Wednesday over the Bahamas as a tropical depression. By Thursday it was a Category 1 hurricane with 80-mph winds, flooding neighborhoods in South Florida and leaving more than 1 million homes and businesses without electricity. The storm then moved over the Gulf of Mexico and, nourished by warm waters, angled toward the Gulf Coast as it steadily rose in intensity.

Fred Barbash and Christopher Lee reported from Washington. Washington Post staff writer Ylan Q. Mui contributed to this story from Washington.