NEW YORK – In the most devastating terrorist onslaught ever waged against the United States, knife-wielding hijackers crashed two airliners into the World Trade Center on Tuesday, toppling its twin 110-story towers. The deadly calamity was witnessed on televisions across the world as another plane slammed into the Pentagon, and a fourth crashed outside Pittsburgh.

“Today, our nation saw evil,” President Bush said in an address to the nation Tuesday night. He said thousands of lives were “suddenly ended by evil, despicable acts of terror.”

Said Adm. Robert J. Natter, commander of the U.S. Atlantic Fleet: “We have been attacked like we haven’t since Pearl Harbor.”

Establishing the U.S. death toll could take weeks. The four airliners alone had 266 people aboard and there were no known survivors. Sources said that about 100 people were believed dead at the Pentagon, but NBC News, quoting other Pentagon sources, listed the casualties there at more than 800.

In addition, a firefighters union official said he feared half of the 400 firefighters who first reached the scene had died in rescue efforts at the World Trade Center – where 55,000 people worked – and dozens of police officers were believed missing.

“The number of casualties will be more than most of us can bear,” a visibly distraught Mayor Rudolph Giuliani said.

“We have entire companies that are just missing,” said firefighters union vice president Mike Carter. “We lost chiefs. … We’re going to have to bury a lot of people.”

A police source said some people trapped in the twin towers managed to call authorities or family members, but it was not clear how many people or when all the calls were made. In one of the calls, which took place in the afternoon, a businessman called his family to say he was trapped with police officers, whom he named, the source said.

No one took responsibility for the attacks that rocked the seats of finance and government. But federal authorities identified Osama bin Laden, who has been given asylum by Afghanistan’s Taliban rulers, as the prime suspect.

Aided by an intercept of communications between his supporters and harrowing cell phone calls from at least one flight attendant and two passengers aboard the jetliners before they crashed, U.S. officials began assembling a case linking bin Laden to the devastation. [Story on Page A7]

U.S. intelligence intercepted communications between bin Laden supporters discussing the attacks on the World Trade Center and Pentagon, according to Utah Sen. Orrin Hatch, the top Republican on the Senate Judiciary Committee.

The people aboard planes who managed to make cell phone calls each described similar circumstances: They indicated the hijackers were armed with knives, in some cases stabbing flight attendants. The hijackers then took control of the planes.

At the World Trade Center, the dead and the doomed plummeted from the skyscrapers, among them a man and woman holding hands.

Shortly after 7 p.m., crews began heading into ground zero of the attack to search for survivors and recover bodies. All that remained of the twin towers by then was a pile of rubble and twisted steel that stood barely two stories high, leaving a huge gap in the New York City skyline.

“Freedom itself was attacked this morning and I assure you freedom will be defended,” said Bush, who was in Florida at the time of the catastrophe. As a security measure, he was shuttled to a Strategic Air Command bunker in Nebraska before leaving for Washington.

“Make no mistake,” he said. “The United States will hunt down and pursue those responsible for these cowardly actions.”

More than nine hours after the U.S. attacks began, explosions could be heard north of the Afghan capital of Kabul, but American officials said the United States was not responsible. “It isn’t us. I don’t know who’s doing it,” Pentagon spokesman Craig Quigley said.

Officials across the world condemned the attacks but in the West Bank city of Nablus, thousands of Palestinians celebrated, chanting “God is Great” and handing out candy. The United States has become increasingly unpopular in the Mideast in the past year of Israeli-Palestinian fighting, with Washington widely seen as siding with Israel against the Arab world.

At the Pentagon, the symbol and command center for the nation’s military force, one side of the building collapsed as smoke billowed over the Potomac River. Rep. Ike Skelton, briefed by Pentagon officials, said, “There appear to be about 100 casualties” in the building.

The first airstrike – on the World Trade Center – occurred shortly before 8:45 a.m. EDT. A burning, 47-story part of the trade center complex, long since evacuated, collapsed in flames just before nightfall.

Emergency Medical Service worker Louis Garcia said initial reports indicated that bodies were buried beneath the 2 feet of soot on streets around the trade center.

“A lot of the vehicles are running over bodies because they are all over the place,” he said.

Said National Guard member Angelo Otchy of Maplewood, N.J., “I must have come across body parts by the thousands. I came across a lady, she didn’t remember her name. Her face was covered in blood.”

For the first time, the nation’s aviation system was completely shut down as officials considered the frightening flaws that had been exposed in security procedures. Financial markets were closed, too.

Top leaders of Congress were led to an undisclosed location, as were key officials of the Bush administration. Guards armed with automatic weapons patrolled the White House grounds and military aircraft secured the skies above the capital city. National Guard troops appeared on some street corners in the nation’s capital.

Evacuations were ordered at the tallest skyscrapers in several cities, and high-profile tourist attractions closed – Walt Disney World, Mount Rushmore, Seattle’s Space Needle, the Gateway Arch in St. Louis.

The Federal Reserve, seeking to provide assurances that the nation’s banking system would be protected, said it would provide additional money to banks if needed.

In Afghanistan, where bin Laden has been given asylum, the nation’s hardline Taliban rulers rejected suggestions he was responsible.

Bin Laden came to prominence fighting alongside the U.S.-backed Afghan mujahedeen – holy warriors – in their war against Soviet troops in the 1980s. But former followers say he turned against the United States during the 1991 Gulf War, seething at the deployment of U.S. troops in Saudi Arabia during the Gulf War campaign to oust Iraq from Kuwait. He has repeatedly called on Muslims worldwide to join in a jihad, or holy war, against the United States.

Abdel-Bari Atwan, editor of the Al-Quds al-Arabi newspaper, said he received a warning from Islamic fundamentalists close to bin Laden, but had not taken the threat seriously. “They said it would be a huge and unprecedented attack, but they did not specify,” Atwan said in a telephone interview in London.

Eight years ago, the World Trade Center was a terrorist target when a truck bomb killed six people and wounded about 1,000 others. Just the death toll on the planes alone could surpass the 168 people killed in the 1995 bombing of the federal building in Oklahoma City.

This is how Tuesday’s mayhem unfolded:

At about 8:45 a.m., a hijacked airliner crashed into the north tower of the trade center, the 25-year-old, glass-and-steel complex that was once the world’s tallest.

Clyde Ebanks, an insurance company vice president, was at a meeting on the 103rd floor of the south tower when his boss said, “Look at that!” He turned to see a plane slam into the other tower.

“I just heard the building rock,” said Peter Dicerbo, a bank employee on the 47th floor. “It knocked me on the floor. It sounded like a big roar, then the building started swaying. That’s what really scared me.”

The enormity of the disaster was just sinking in when 18 minutes later, the south tower also was hit by a plane.

“All this stuff started falling and all this smoke was coming through. People were screaming, falling and jumping out of the windows,” said Jennifer Brickhouse, 34, from Union, N.J.

The chaos was just beginning. Workers stumbled down scores of flights, their clothing torn and their lungs filled with smoke and dust.

John Axisa said he ran outside and watched people jump out of the first building; then there was a second explosion, and he felt the heat on the back of his neck.

Donald Burns, 34, was being evacuated from the 82nd floor when he saw four people in the stairwell. “I tried to help them but they didn’t want anyone to touch them. The fire had melted their skin. Their clothes were tattered,” he said.

Worse was to come. At 9:50, one tower collapsed, sending debris and dust cascading to the ground. At 10:30, the other tower crumbled.

Glass doors shattered, police and firefighters ushered people into subway stations and buildings. The air was black, from the pavement to the sky. The dust and ash were inches deep along the streets.

Bridges and tunnels were closed to all but pedestrians. Subways were shut down for much of the day; many commuter trains were not running.

Meanwhile, at about 9:30 a.m., an airliner hit the Pentagon – the five-sided headquarters of the American military. “There was screaming and pandemonium,” said Terry Yonkers, an Air Force civilian employee at work inside the building.

The military boosted security across the country to the highest levels, sending Navy ships to New York and Washington to assist with air defense and medical needs.

A half-hour after the Pentagon attack, a United Airlines Flight 93, a Boeing 757 jetliner en route from Newark, N.J., to San Francisco, crashed about 80 miles southeast of Pittsburgh.

Airline officials said the other three planes that crashed were American Airlines Flight 11, a Boeing 767 from Boston to Los Angeles, apparently the first to hit the trade center; United Airlines Flight 175, also a Boeing 767 from Boston to Los Angeles, which an eyewitness said was the second to hit the skyscrapers; and American Airlines Flight 77, a Boeing 757 en route from Washington-Dulles to Los Angeles that a source said hit the Pentagon.

“We’re at war,” said Gaillard Pinckney, an employee at the Housing and Urban Development office in Columbia, S.C. “We just don’t know with who.”

Giuliani said it was believed the effects of the plane crashes eventually brought the buildings down, not planted explosive devices.

Hyman Brown, a University of Colorado civil engineering professor and the construction manager for the World Trade Center, speculated that flames fueled by thousands of gallons of aviation fuel melted steel supports.

“This building would have stood had a plane or a force caused by a plane smashed into it,” he said. “But steel melts, and 24,000 gallons of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire.”

At midafternoon, Giuliani said 1,500 “walking wounded” had been shipped to Liberty State Park in New Jersey by ferry and tugboat, and 750 others were taken to New York City hospitals, among them 150 in critical condition.

Well into the night, a steady stream of boats continued to arrive in the park. “Every 10 minutes another boat with 100 to 150 people on it pulls up,” said Mayor Glenn Cunningham. “I have a feeling this is going to go on for several days.”

Felix Novelli, who lives in Southampton, N.Y., was in Nashville with his wife for a World War II reunion. He was trying to fly home to New York when the attacks occurred

“I feel like going to war again. No mercy,” he said. “This is Dec. 7th happening all over again. We have to come together like ’41, go after them.”

The attack on Pearl Harbor claimed the lives of 2,390 Americans, most of them servicemen.

How to assist victims

Numbers to call to offer assistance to victims of Tuesday’s terrorist attacks:

. American Red Cross, blood or cash donations: (800) HELP-NOW

. Salvation Army, cash donations: (800) SAL-ARMY

. Medical personnel who wish to donate their services should contact their local American Red Cross office. The Federal Emergency Management Agency does not accept donations, but coordinates volunteer agencies at disaster sites.