Terrorists unleashed an astonishing air assault on America's military and financial power centers yesterday morning, hijacking four commercial jets and then crashing them into the World Trade Center in New York, the Pentagon and the Pennsylvania countryside.

There were no reliable estimates last night of how many people were killed in the most devastating terrorist operation in American history. The number was certainly in the hundreds and could be in the thousands.

It was the most dramatic attack on American soil since Pearl Harbor, and it created indelible scenes of carnage and chaos. The commandeered jets obliterated the World Trade Center's twin 110-story towers from their familiar perch above Manhattan's skyline and ripped a blazing swath through the Defense Department's imposing five-sided fortress, grounding the domestic air traffic system for the first time and plunging the entire nation into an unparalleled state of anxiety.

U.S. military forces at home and abroad were placed on their highest state of alert, and a loose network of Navy warships was deployed along both coasts for air defense.

The terrorists hijacked four California-bound planes from three airports on the Eastern Seaboard; the airliners were loaded with the maximum amount of fuel, suggesting a well-financed, well-coordinated plot. First, two planes slammed into the World Trade Center. Then an American Airlines plane out of Dulles International Airport ripped through the newly renovated walls of the Pentagon, perhaps the world's most secure office building. A fourth jet crashed 80 miles southeast of Pittsburgh, shortly after it was hijacked and turned in the direction of Washington.

None of the 266 people aboard the four planes survived. There were even more horrific but still untallied casualties in the World Trade Center and the Pentagon, which together provided office space for more than 70,000 people. At just one of the firms with offices in the World Trade Center, the Marsh & McLennan insurance brokerage, 1,200 of its 1,700 employees were unaccounted for last night.

The spectacular collapse of the Trade Center's historic twin towers and another less recognizable skyscraper during the rescue operations caused even more bloodshed. At least 300 New York firefighters and 85 police officers are presumed dead. The preliminary list of victims included the conservative commentator Barbara K. Olson, "Frasier" executive producer David Angell and two hockey scouts from the Los Angeles Kings.

No one claimed responsibility for the attacks, but federal officials said they suspect the involvement of Islamic extremists with links to fugitive terrorist Osama bin Laden, who has been implicated in the 1998 bombings of two U.S. embassies in Africa and several other attacks. Law enforcement sources said there is already evidence implicating bin Laden's militant network in the attack, and politicians from both parties predicted a major and immediate escalation in America's worldwide war against terrorism.

In a grim address to the nation last night, President Bush denounced the attacks as a failed attempt to frighten the United States, and promised to hunt down those responsible. "We will make no distinction," he said, "between the terrorists who committed these acts and those who harbor them."

Bush vowed that America would continue to function "without interruption," and federal offices and Congress are scheduled to be open today. But the New York Stock Exchange and Nasdaq Stock Market will remain closed, along with most businesses in lower Manhattan. And yesterday was a day of extraordinary interruptions -- for the president, for federal Washington and for the country.

Bush was in a classroom in Florida yesterday morning when the attacks began and spent the day on the move for security reasons, flying to military bases in Louisiana and then Nebraska before returning to Washington in the evening. At one point at Barksdale Air Force Base in Louisiana, the president rode in a camouflaged, armored Humvee, guarded by machine gun-toting soldiers in fatigues.

Vice President Cheney and first lady Laura Bush were whisked away to undisclosed locations in the morning, and congressional leaders were temporarily moved to a secure facility 75 miles west of Washington. The White House, the Capitol, the Supreme Court, the State Department and the Treasury Department were evacuated, along with federal buildings nationwide and the United Nations in New York.

Private buildings also were shut down, from the Space Needle in Seattle to the Sears Tower in Chicago to Walt Disney World in Orlando. America's borders with Canada and Mexico were sealed. New York's mayoral primary was abruptly postponed. So was Major League Baseball's schedule for the night.

Wireless networks buckled under the barrage of cell phone calls. The besieged Internet search engine Google told Web surfers to try radio or TV instead. Amtrak train and Greyhound bus operations were also halted in the Northeast.

Last night, fires were still burning amid the rubble of the World Trade Center, and pools of highly flammable jet fuel continued to hinder rescue teams searching through waist-deep rubble.

The Federal Emergency Management Agency dispatched eight search-and-rescue teams to New York and four teams to the Pentagon. The Department of Health and Human Services sent medical teams and mortuary teams, and activated a national medical emergency cadre of 7,000 volunteers for the first time.

The Empire State Building went dark as a symbol of national mourning. In Washington, Republicans and Democrats presented a united front in condemning the attacks; members of Congress delivered a spontaneous rendition of "God Bless America" after a news conference on the Capitol steps.

"We are outraged at this cowardly attack on the people of the United States," the leaders of Congress said in a bipartisan statement. "Our heartfelt prayers are with the victims and their families, and we stand strongly united behind the President as our commander-in-chief."

The impact of the attacks reverberated not just in the United States but in every major capital. European and Asian airlines canceled all flights to the United States and recalled or diverted those already in the air. Flights over London, Paris and other capitals were re-routed over less populous areas. London's financial district was largely evacuated; security was bolstered around U.S. schools and embassies in many countries.

Panic buying caused oil and gold prices to soar while stock investors in all major foreign markets dumped shares in the most frenzied wave of selling since the 1987 crash. In the Middle East, China and the Yugoslav republic of Serbia, some people welcomed the attacks, but an array of international leaders pledged support for the victims.

Israeli Prime Minister Ariel Sharon condemned the attack in blistering terms, and described it as a "turning point" in the global war against terrorism. Palestinian leader Yasser Arafat condemned the attack as well, although some Palestinians in Israeli-occupied territories and Lebanon celebrated with glee.

But amid all the sadness and all the outrage, there were questions about lax security and inadequate intelligence, as Americans tried to fathom how such a catastrophe could happen with no apparent warning. On at least two of the airliners, according to federal officials, the hijackers were armed with nothing but knives. How did they get away with it?

In fact, counterterrorism experts have talked in recent years about cyber-attacks and biological attacks. Security officials issued warnings just last month about bin Laden's threats to American installations abroad.

But yesterday's attacks caught a vast security apparatus off guard. The military command center in Colorado's Cheyenne Mountain, responsible for U.S. air defenses, received word just 10 minutes before the first aircraft struck the World Trade Center that a American plane had been hijacked. The notification came too late for fighter jets to take action, a senior Air Force officer said.

The disaster began to unfold at 8:48 a.m., when American Airlines Flight 11, carrying 92 people from Boston to Los Angeles, crashed into the North Tower of the World Trade Center, the landmark glass-and-steel complex at the southern tip of Manhattan that provided office space for 50,000 workers. Islamic militants had detonated a bomb there in 1993, killing six people. Yesterday's terrorism turned out to be far worse.

Eighteen minutes later, United Airlines Flight 175, carrying 65 people on the same Boston-to-Los Angeles route, tore through the South Tower with an even larger explosion. The collisions shrouded New York's helter-skelter financial district in pallid ash, and created mass pandemonium inside and outside the towers. Workers were screaming, running for stairways, gasping for air. Several of them began leaping to their death from the upper floors.

But the scene soon shifted from America's financial mecca to its military fortress. At about 9:40 a.m., American Airlines Flight 77, carrying 64 people from Dulles to Los Angeles, barreled into the west wing of the Pentagon in yet another fiery collision, destroying at least four of the five rings that encircle the world's largest office building. A Pentagon spokesman called the casualties "extensive," although they were clearly not as extensive as New York's.

The Federal Aviation Administration promptly banned takeoffs nationwide, ordered domestic flights to land at the nearest airport and diverted international flights to Canada. But officials soon confirmed that a fourth plane, United Airlines Flight 93, carrying 45 people from Newark to San Francisco, had crashed in Shanksville, Pa. It had been hijacked as well -- one passenger called 911 from a cell phone -- and had been heading toward Washington when it went down.

Then it was back to the World Trade Center. Shortly before 10 a.m., the South Tower collapsed with an earthshaking roar. Smoke replaced steel as if the building had suddenly imploded. A half-hour later, the North Tower collapsed. Mayor Rudolph Giuliani publicly urged New Yorkers to stay calm and stay put -- unless they were below Canal Street in lower Manhattan.

"If you're south of Canal Street, get out," he warned. "Just walk north."

America's battle against terrorism, it seemed clear last night, will never be the same. The nation's airports are expected to reopen at noon today, but with beefed-up security measures: no more curbside check-in, and a possible return of armed "air marshals" to prevent future hijackings.

Many members of both parties declared that for all practical purposes, the nation is at war. At a briefing last night in the battered Pentagon, Defense Secretary Donald H. Rumsfeld warned that America's enemies should not rest easy.

For now, those enemies have not been publicly identified. But government officials said they have strong evidence from multiple sources linking the attacks to bin Laden and his terrorist web, known as al Qaeda.

Journalists with access to bin Laden said his followers have been boasting about preparations for major attacks against the United States in retaliation for American support of Israel. Bin Laden has already been linked to the 1993 bombing of the World Trade Center, the 1998 embassy bombings in Kenya and Tanzania, and last year's attack on the USS Cole in Yemen. Yesterday, government officials said they intercepted messages from bin Laden associates gloating about hitting their targets.

Before the mayhem, though, U.S. intelligence all pointed to an attack overseas. The State Department had warned travelers in an advisory Friday, and U.S. military and diplomatic posts abroad have been on alert as well.

Terrorism experts have repeatedly warned that U.S. airport security is extremely lax, warnings that have been backed up by a stack of studies. When Department of Transportation investigators tried to breach security at eight airports three years ago, they succeeded 68 percent of the time.

"The security of airports is pathetic," said Harvey W. Kushner, a Long Island University professor and terrorism consultant to several federal agencies. "It's very easy to have someone get on a plane and wreak havoc."

Today, at least, the debates over education, health care, Social Security and the budget surplus that have consumed Washington in recent months have been put on hold; perhaps for the first time since the Gulf War, national security is at the top of the agenda. Rep. Curt Weldon (R-Pa.) was preparing to call for more military spending at a news conference at the time of the attacks.

"This is a failure of the U.S. intelligence system, caused by a lack of resources and by complacency," he said. "Today, our government failed the American public."

But that was a discordant note yesterday in Washington, where solidarity was the watchword of the day. In his speech last night, Bush emphasized the nation's harmony, noting that "a great people have been moved to defend a great nation." After reading from the 23rd Psalm, he proclaimed that even amid suffering and death, Americans will remain committed to their freedom-loving way of life.

"This is a day when all Americans from every walk of life unite in our resolve for justice and peace," he said. "America has stood down enemies before, and we will do so this time."

Minutes after an American Airlines plane crashed into the World Trade Center in New York City, a United airliner is about to hit the complex.Firefighters battle blazes at the Pentagon, which was hit by a plane that had been hijacked after taking off from Dulles International Airport.A fireball explodes from World Trade Center, which was hit by two airliners. The center's twin towers later collapsed.Firefighter Michael Saber drinks from a hydrant in the rubble of the World Trade Center. Many New York firefighters were missing and feared dead.New Yorkers walking through lower Manhattan encounter thick dust after the mid-morning collapse of two towers in the World Trade Center.