Hurricane Katrina barreled into the Gulf Coast on Monday morning, its fierce winds cutting a 125-mile swath of destruction stretching from coastal Alabama across Mississippi to the French Quarter and the Superdome. At least 55 people were killed.

The storm's leading edge, wielding winds up to 145 mph across the Gulf of Mexico, made landfall as a fearsome Category 4 hurricane at 7:10 a.m. Eastern time near the Louisiana bayou town of Buras, about 63 miles southeast of New Orleans.

Katrina then wheeled into western Mississippi, bringing a 20-foot storm surge along the coast near Biloxi. It headed north later Monday to inundate most of the Mississippi Delta, spun off dozens of tornadoes through the South and promised drenching rains later in the week as far north as the Ohio Valley.

By late afternoon Monday the storm was downgraded to Category 1, with winds of 95 mph. But authorities in Louisiana, Mississippi and Alabama, forced to hunker down for most of the day, were only beginning to take stock of the damage.

"The high water is keeping us out of some areas, and the high winds are keeping our aircraft grounded," Louisiana Gov. Kathleen Babineaux Blanco (D) told reporters at a Baton Rouge news conference. She said she had "isolated reports of between four and six" fatalities, but nothing confirmed. Nonetheless, she said grimly, "we're worried."

Mississippi Gov. Haley Barbour (R) said Katrina struck his state "like a ton of bricks" and he feared "a lot of dead people down there" on the coast. "We know some people got trapped, and we pray they are okay," Barbour told reporters in Jackson.

In Mississippi's Harrison County, an emergency official confirmed 50 hurricane-related deaths Monday night, most at an apartment complex in Biloxi, the Associated Press reported.

President Bush pledged emergency aid to the afflicted states, and officials said he was likely to tap the 700 million-barrel petroleum stockpile to help Gulf Coast oil companies forced by Katrina to shut down.

"Our Gulf Coast is getting hit and hit hard," Bush said during a visit to El Mirage, Ariz. "I want the folks there on the Gulf Coast to know that the federal government is prepared to help you when the storm passes. . . . In the meantime, America will pray . . . for the health and safety of all our citizens."

New Orleans, with its combination of high population density and low-lying terrain, figured to be devastated by Katrina, and some experts predicted the storm could become one of the worst catastrophes in U.S. history.

But the city managed to avoid the worst of the worst. The Mississippi River did not breach New Orleans's famed levees to any serious degree, at least in part because Katrina veered 15 miles eastward of its predicted track just before landfall.

"We believe we were spared," said Jacquie Bauer, a spokesman for Jefferson Parish, La., which lies next to New Orleans. Still, she said, some rescue crews had come back saying "the damage was worse than anything they've seen before."

And no one was claiming victory over Katrina or telling hundreds of thousands of people who evacuated the city that the danger was past. "The roads are flooded, there is no electricity, the phones are down, there is no food or water," Blanco said. "I have ordered the Louisiana State Police to block reentry to affected areas. Wherever you live, it is still too dangerous for people to return home."

Blanco said the storm had breached a 50-inch New Orleans water main, rendering the city's water undrinkable in all but a few neighborhoods. She said eastern New Orleans and St. Bernard Parish east of the city had been "devastated by floodwaters and high winds."

Late Monday, the extent of damage to New Orleans could still only be guessed at. Its proudest downtown high-rises were scarred by scores of broken windows. Curtains and blinds billowed outward while wind and rain poured into hotel rooms and offices inside.

"We prayed and sang and read the Word -- and God was with us," said Jo Hardeman, 53, a tourist from Nashville, who was unable to get out of the city because of the closed airport and was forced to spend the hurricane inside the Fairmont Hotel.

Katrina peeled away half of the golden metal roof fabric covering the Superdome, opening two holes in its skin. That allowed water to fall 19 stories, sprinkling 9,000 refugees sweltering without air conditioning in the shelter below.

New Orleans's graceful avenues were littered with everything from downed trees to shattered glass. St. Bernard Parish reportedly had 40,000 flooded homes.

Mississippi may have fared even worse. "Let me tell you something, folks: I've been out there. It's complete devastation," Gulfport Fire Chief Pat Sullivan told AP.

Storm surges as high as 28 feet were reported in the state, a Knight-Ridder Newspapers dispatch said. Harrison County's command post had to be moved from the Gulfport courthouse because of rising water.

Elsewhere along the Mississippi coast, the storm pushed water up to the second floor of homes, flooded floating casinos near Biloxi, uprooted hundreds of trees and flung sailboats across a highway, the AP said.

In Alabama, Katrina's arrival was marked by the flash and crackle of exploding transformers. The hurricane toppled huge oak branches on Mobile's waterfront and broke apart an oil-drilling platform, the AP said. Muddy six-foot waves crashed into the eastern shore of Mobile Bay, flooding stately, antebellum mansions and littering them with oak branches.

The hurricane all but paralyzed energy production all along the coast, one of the nation's oil-and-gas hubs. It battered refineries, offshore oil platforms and pipelines, raising fears that already high oil prices could reach debilitating heights in coming weeks.

More than 700 oil platforms had been evacuated by late Monday. Energy companies warned they would need several days to assess damage to major facilities. Electronic trading of crude oil rose nearly $5 Monday to peak at a record $70.80 a barrel.

Katrina was a marginal hurricane with peak winds of 95 mph when it plowed into Florida's Atlantic coast last week. Nonetheless, it was blamed for 11 deaths and caused extensive flooding near Miami and torrential rains in the Keys before spinning into the Gulf of Mexico and turning north.

There Katrina gained strength and definition, unimpeded by land masses that trigger the crosswinds that spread storms out and eventually rip them apart. "The warm Gulf waters provide extra octane," National Hurricane Center meteorologist Chris Sisko explained in a telephone interview from Miami.

By Sunday, Katrina was a Category 5 hurricane -- winds in excess of 155 mph. Experts were warning of apocalyptic damage to New Orleans, a city of 485,000 people, most of whom live below sea level.

Mayor Ray Nagin ordered a mandatory evacuation of New Orleans over the weekend, and an estimated 80 percent of residents complied. Those who remained behind were mostly visitors and tourists trapped because the airport had closed.

These stayed in hotels or went to emergency shelters. The biggest of these was the Superdome, but the American Red Cross also had about 30,000 people in shelters across the state.

Fortunately for New Orleans, Katrina peaked too soon. "When a storm becomes stronger, it pushes larger waves," said David S. Nolan, a meteorology professor at the University of Miami. The water temperature drops, and "the available energy starts to decline," he added.

Katrina dropped from a Category 5 to a strong Category 4, then "wobbled" slightly just before landfall, veering east from New Orleans when it hit the coast. Even so, National Weather Service Director David L. Johnson said, Katrina is quite likely to end up among the 10 strongest hurricanes to hit the continental United States.

"The city of New Orleans was fortunate in that the eye passed very slightly to the east," Johnson said in a telephone interview. "Because of the wind's circulation, it's the northeast quadrant that piles up the water, and had the track been even slightly west of north, the city would have been devastated."

Gugliotta reported from Washington. Staff writer Jonathan Weisman in Washington contributed to this report.

In the aftermath of Hurricane Katrina, David Diaz examines the damage to the Sadler Apartments on the waterfront in Biloxi, Miss., where he and his brother had lived. A Red Cross truck is among the flooded vehicles idled in front of a hotel just off Interstate 10 in Pascagoula, Miss. Gov. Haley Barbour said Hurricane Katrina hit the state "like a ton of bricks."Fire and rescue personnel launch a boat amid floodwaters from Hurricane Katrina as they try to reach a family outside a motel in Pascagoula, Miss. Storm surges of at least 20 feet were reported along the Mississippi coast.