It was the worst attack on American soil since the Japanese bombing of Pearl Harbor on Dec. 7, 1941 and gave the country a new date that will live in infamy.
The twin towers which symbolize U.S. financial clout were toppled and the Pentagon, the nerve-center of the nation's military might, severely damaged.
The attack forced the first mandatory evacuation of the U.S. Capitol and drove congressional leaders into safe but secret locations while it drew bipartisan vows of retaliation.
No group took immediate responsibility for the attack but suspicions centered on an implacable U.S. foe - exiled Saudi Osama bin Laden, whose followers were held responsible for murderous attacks on U.S. embassies in Africa.
Explosions lit up the night sky in Kabul, Afghanistan, the nation where bin Laden is believed to live and reports said there were missiles flying across the city. A senior Pentagon official denied U.S. involvement and responsibility for the attack was laid at the feet of an Afghan opposition group.
'HORRIFIC' - SAYS MAYOR GIULIANI
New York Mayor Rudolph Giuliani said the death toll could ultimately be "horrific" and that the city's hospitals were swamped with casualties. Hundreds of firefighters and police are missing and feared dead after trying to rescue others.
The entire nation was brought to a halt by scenes of terrified people fleeing the mayhem flashing across TV screens. The mighty twin towers, anchoring the southern tip of Manhattan imploded one at a time, sending a massive plume of dust and smoke billowing over the city.
Officials feared the death toll could climb into the thousands - perhaps tens of thousands - as 40,000 people alone worked in the steel and glass Trade Center towers and a nearby 47-story building, World Trade Center No. 7, which collapsed seven hours later after a raging fire.
World leaders condemned the attack. German Chancellor Gerhard Schroeder called the strikes a "declaration of war against the civilized world."
The attacks presented President Bush with the toughest test of his eight-month presidency. He cut short a trip to Florida and flew to Offutt Air Force Base in Nebraska, after stopping briefly in Louisiana where he vowed to bring those responsible to justice. Aides said he planned to address the nation from Washington later on Tuesday.
The attacks, which involved the hijacking of four commercial planes - two from Boston, one each from Newark and Dulles, outside Washington - brought normal life across the country to a standstill, turning major cities into eerie ghost towns. Attorney General John Ashcroft said that least one of the planes was commandeered by hijackers armed with knives.
All financial markets were closed and millions of workers sent home early. All commercial flights were canceled and all airports shut in an unprecedented move. For the first time since D-Day, major league baseball games were canceled.
KAMIKAZE ATTACK ON PENTAGON
The crisis began shortly before 9 a.m. when the first plane slammed into the north tower in the heart of New York's financial district, opening a massive hole near the top. A second plane followed 15 minutes later, scoring a direct hit on the south tower.
Minutes later came the report of a third kamikaze attack on the Pentagon, in Northern Virginia across the Potomac from Washington. That building, too, burst into flames.
Then came the deadliest blow of all as first one and then the second of New York's twin towers collapsed with a roar in a burst of smoke, fire and metal.
The towers - which opened for business in 1975 - briefly claimed the title of the world's tallest buildings but soon surpassed by the Sears Tower in Chicago. But the twin towers became one of New York's best-known landmarks, rivaling the Statue of Liberty and the Empire State Building.
Not knowing if more attacks were on the way, authorities evacuated key landmarks like the White House, the fire-damaged Pentagon, the Sears Tower in Chicago, the CIA building and the Walt Disney theme parks.
It was the worst attack on American soil since Japanese war planes bombed the U.S. fleet at Pearl Harbor on Dec. 7, 1941, killing 2,280 soldiers and 68 civilians and forcing the United States into World War Two.
Former Secretary of State Henry Kissinger told CNN, "This is comparable to Pearl Harbor and we must have the same response and the people who did it must have the same end as the people who attacked Pearl Harbor."
266 ABOARD FOUR HIJACKED PLANES
Authorities said at least 266 people were on board four hijacked planes - two that crashed into the twin towers, one that slammed into the Pentagon and a fourth that crashed in a wooded area near Pittsburgh, Pennsylvania.
"I looked outside and saw a big chunk of the World Trade Center missing," said Verizon employee Ellen Leon. "Fifteen minutes later I saw people jumping out of the building. Bodies were flying out. I don't know if they were already dead or if they were just going to die."
The attacks triggered scenes of panic, disbelief and heroism in the largest U.S. city, where police and firefighters risked their lives to save people from the twin towers before its 200,000 tons of steel frame and 43,000 windows came smashing down, covering lower Manhattan in a shower of soot.
"It's nuts, there is debris and dust everywhere, and it looks as though a volcano erupted down there," said Michael DeVita, who was working on the 84th floor of World Trade Center Building No. 2 when the first plane hit Building No. 1.
"Hundreds of people are burned from head to toe," said Dr. Steven Stern at St. Vincent's Hospital in the Greenwich Village neighborhood of lower Manhattan. Rescue workers used commuter ferries to carry victims across the Hudson River to safety in Hoboken, New Jersey, where the scene resembled a war zone, with victims laid out on stretchers, limping on crutches, and others walking without a shirt and with their pants torn.
BUSH VOWS VENGEANCE
Bush vowed to bring those responsible for the attacks to justice as he stopped in Louisiana to talk to the nation. "Make no mistake, the United States will hunt down and punish those responsible for these cowardly acts," the president said.
He had begun his day in Florida prepared to speak about education but quickly changed his plans when the news of the attacks emerged.
With a military jet at each wingtip, Bush's Air Force One aircraft rushed to Barksdale Air Force Base in Louisiana and was immediately surrounded by soldiers bearing automatic weapons and dressed in camouflage fatigues.
He was on the ground there only briefly before heading for Nebraska, landing at Offutt, a base with a key role in strategic nuclear planning under its U.S. Strategic Command.
Disaster relief agencies said they were working with the military to rush thousands of pints of blood to New York City and Washington to treat an untold number of injuries from aerial hijack attacks in those cities.
Foreign financial markets fell sharply on news of the attacks. The London FTSE index plummeted 5.7 percent and Latin American markets tumbled. Oil prices spiked up. Palestinian leader Yasser Arafat condemned the attack but some Palestinians in the Israeli-occupied territories and in Lebanon celebrated.
Bin Laden, a Saudi millionaire and Islamic militant, was blamed for the 1998 bombings of two U.S. embassies in Africa in which 224 people died.
An Arab journalist with access to bin Laden told Reuters in London the renegade Saudi had warned three weeks ago of an "unprecedented attack" on U.S. interests.