n what appeared to be parallel attacks on quintessential symbols of American financial and military power, hijackers flew jetliners into both towers of the World Trade Center in Manhattan and, less than an hour later, into the Pentagon, outside Washington.

The 110-story towers at the World Trade Center soon collapsed in a horrific storm of flying glass and rubble. The largest of several smaller buildings in the World Trade Center complex, a 47-story structure that had been set ablaze by debris in the morning, gave way in late afternoon.

One wall of the Pentagon -- the fortress-like headquarters of the Defense Department across the Potomac River from Washington that was built at the beginning of World War II -- also tumbled to the ground.

President Bush said tonight that thousands of people had died in the attacks.

Mayor Rudolph W. Giuliani said at around 10:30 p.m. that outside of the airplane passenger deaths, there were six confirmed fatalities in New York, "and tragically there are going to be a lot more than that, but that's what we know of at this point." In all, 266 people perished in the four planes that were hijacked, including one that went down in Pennsylvania, with its apparently intended target being Camp David, the presidential retreat in Maryland.

President Bush, in a televised address to the nation this evening, denounced those responsible for the attack and promised swift and harsh justice. He also pledged the nation's aid to the victims.

"Today our nation saw evil, the very worst of human nature, and we responded with the best of America," President Bush said.

"The search is under way for those who are behind these evil acts," he added. "We will make no distinction between the terrorists who committed these acts and those who harbor them."

Mayor Giuliani, who appeared on television tonight visibly shaken because many high-ranking fire and police officials had been reported dead or missing, said that "tomorrow the effort will be at trying to recover as many people as possible and trying to clean up the horrible mess that was created by all of this."

Across the country, emergency procedures that were never used during the cold war were suddenly invoked, not as drills but for real. For the first time in history, all civilian airplane flights were grounded while military and civilian officials conferred by telephone from secure locations.

At the Pentagon, still smoldering, Defense Secretary Donald H. Rumsfeld said, "The Pentagon is functioning. It will be in business tomorrow." He and Attorney General John D. Ashcroft condemned the attacks and pledged that their agencies would do everything possible to bring the organizers to justice.

No one immediately claimed responsibility for the attacks. But the extraordinary planning required, the hijackers' apparent familiarity with the jetliners they commandeered, and the history of attacks on American targets in recent years all led to speculation that this attack was directed by Osama bin Laden, the Islamic militant believed to operate out of Afghanistan. Afghanistan's hard-line Taliban rulers rejected the idea, but American officials saw that as a defensive measure.

One passenger on the plane that smashed into the Pentagon was Barbara Olson, whose husband, Solicitor General Theodore Olson, said she had called him twice from a cell phone twice before the crash. He said that she had been scheduled to take the same flight on Monday, but had delayed her trip a day to celebrate his birthday with him on Tuesday morning.

Mrs. Olson -- a former congressional investigator and aide to the Senate minority whip, Don Nickles, who has gone on to be a commentator for CNN -- said that the passengers, the pilot and the rest of the flight crew, had been herded to the back of the plane. Mr. Olson said the only weapons she mentioned were knives and cardboard cutters.

In New York, people watched in disbelief as first one tower and then the other appeared to explode, floor by floor. Then a debris-laden avalanche began falling, blocking out the brilliance of the late-summer sun and covering the streets of lower Manhattan in a ghostly gray layer .

President Bush, who was in Sarasota, Fla., when the two planes flew into the World Trade Center around 9 a.m. Eastern time, called the destruction in New York "an apparent terrorist attack on our country" and ordered a full-scale investigation to "hunt down the folks who committed this act." Later, at Barksdale Air Force Base in Louisiana on his way to Nebraska, he said that the government had taken "all appropriate steps to protect the American people."

"Make no mistake," the president said in Louisiana, "the United States will hunt down and punish those responsible for these cowardly acts." Adm. Robert J. Natter, the commander of the Atlantic fleet, ordered aircraft carriers and guided missile destroyers to New York and Washington.

"We have been attacked like we haven't since Pearl Harbor," he said. Admiral Natter also dispatched amphibious ships carrying Marines and sailors who could assist with security and surgical teams that could help treat injured people.

Officials involved in piecing together the sequence of destruction said four jetliners were hijacked and used in the attacks. American Airlines flight 11, a Boeing 767 that had left Boston, hit one tower of the World Trade Center first.

It was followed about 20 minutes later by United Airlines flight 75, also a Boeing 767 diverted from a Boston-to-Los Angeles flight.

American flight 77, a Boeing 757 that had taken off from Dulles International Airport near Washington, went down at the Pentagon, while a United flight from Newark to San Francisco crashed 80 miles southeast of Pittsburgh.

Rep. James Moran of Virginia said after attending a briefing in Washington that the intended target of the plane that crashed in Pennsylvania appeared to be Camp David, the presidential retreat in Maryland. The crash site was 85 miles northwest of there.

In New York, emergency workers were reported to be preparing to ferry bodies across the Hudson River to Jersey City this afternoon.

The Associated Press reported that one Jersey City police officer directing traffic shouted: "Get out of here! We have to bring dead bodies through here!"

Acting Gov. Donald T. DiFrancesco of New Jersey declared a state of emergency, which gave him the power to mobilize the state police, the National Guard and other emergency units. In New York, the State Emergency Management Office opened its emergency operations center in Albany as Governor George E. Pataki called the destruction of the World Trade Center "an attack upon New York, an attack upon America, an attack upon our way of life."