In the blood-stained history of terrorism, there has never been a day as horrifying as September 11, 2001.

In less than an hour, fanatical hijackers struck at the heart of the United States, plunging the nation into a state of shock.

Millions around the world watched on live TV after first one plane then another slammed into the 110-storey twin towers of the World Trade Centre.

Within an hour the most powerful symbols of US economic power had collapsed, ten thousand were feared to have died and the most famous skyline in the world had been changed for ever.

As New York reeled, Washington, the US capital, was under fire. A passenger airliner crashed into the Pentagon, the heart of the US military machine.

Another passenger plane crashed about 80 miles south east of Pittsburgh. A Virginia congressman said its intended target was apparently Camp David, the presidential retreat in Maryland.

An emergency phone operator in Pennsylvania had received a mobile phone call from a man who said he was a passenger locked in the toilet of United Flight 93.

"We are being hijacked, we are being hijacked!" the man said.

As the rescue operation continued last night, another building close to the World Trade Centre collapsed.

In a carefully-planned operation which lethally exposed flaws in US airline security, suicide bombers had seized the four airliners on domestic flights.

The attacks brought the nation to a standstill. Airports were closed, flights were grounded, borders were sealed and jet fighters were scrambled above major cities.

For many it was the nation's greatest humiliation since Pearl Harbour, the attack which triggered US entry into the Second World War.

"This is the second Pearl Harbour. I don't think that I overstate it," said Senator Chuck Hagel.

Chris Yates, an aviation expert at Jane's Transport in London, said: "This is perhaps the most audacious terrorist attack that's ever taken place in the world."

The first strike was against the Trade Centre. A jet smashed high up into one of the towers where tens of thousands of people work every day.

As horrified witnesses described the terrifying scenes, a second jet was filmed by CNN slamming into the second tower lower down, bursting into flames and leaving another gash in the building.

Within an hour, both towers had collapsed, sending clouds of dust billowing down the streets of Manhattan and across the harbour as survivors, many of whom were staggering from the building after the attacks, fled for their lives.

On a school visit in Florida at the time, US President George Bush put the military on high alert.

He said: "Make no mistake. The US will hunt down and punish those responsible for this cowardly act."

The president was flown to the security of Barksdale Air Force Base in Louisiana, then later to the US Strategic Air Command headquarters in Nebraska.

Aides said he would convene a National Security Council meeting by teleconference from there.

The scenes of unbelievable destruction, beamed live around the world on TV, were thought to have been caused by Middle Eastern terrorists.

Abu Dhabi television reported it had received a call from the Democratic Front for the Liberation of Palestine claiming responsibility for crashing two planes into the twin WTC towers, which were the target of Islamic extremist bombers in February 1993. The claim was later denied.

A total of 266 passengers and crew were on board the four jets.

United Airlines said last night it was giving the families of victims on its two lost flights an initial sum of 25,000 dollars (about £18,000) "to help meet their immediate needs".

The airline went on: "United has identified all passengers and crew members onboard these two flights and is in the process of notifying their next of kin and working to assist family members."

United has confirmed the following details:

l UA 93, a Boeing 757 aircraft, departed from Newark, New Jersey at 8.01am local time, bound for San Francisco, with 38 passengers, two pilots, and five flight attendants. The aircraft crashed near Johnstown, Pennsylvania.

l UA 175, a Boeing 767 aircraft, left Boston at 7.58am local time, bound for Los Angeles, with 56 passengers, two pilots, and seven flight attendants. United confirmed the loss of this aircraft. Last radar contact with the aircraft was between Newark and Philadelphia.

In a measure of the carnage on the ground, five hours after the attacks there were no estimates of the overall death toll, but hospitals around New York were struggling to cope with the casualties.

New York Mayor Rudolph Giuliani said 2,100 people were injured - 1,500 "walking wounded" and 600 others who were taken to area hospitals, 150 of them in critical condition.

As shockwaves from the outrage reverberated around the world, Prime Minister Tony Blair called an emergency meeting of the Cobra security committee and announced a range of security measures at British airports.

Mr Blair, who cut short his speech to the TUC, said Britain will stand full square alongside the US in the battle to drive the "new evil" of terrorism from the world.

Key EU officials rushed back to their Brussels headquarters and called emergency meetings to discuss how they can help the US.

Nato also called an urgent meeting of ambassadors from its 19 members to discuss the security implications.

World travel was thrown into chaos. Many US-bound flights had already left the UK when American officials closed all US airports.

Thousands of passengers, including the Duke of York, were diverted, or turned round mid-Atlantic.

In the UK, Queen expressed her "growing disbelief and total shock".

Workers in London left some high-profile buildings amid concern they could be the next target.

The London Stock Exchange, the Lloyds building, Canary Wharf and the NatWest Tower in central London were among those with workers streaming out as panic spread across the world.

Armed police were sent to the US embassy in London's Grosvenor Square. Security was also stepped up at British military bases.

But in the West Bank and Gaza, thousands of Palestinians celebrated the terror attacks chanting "God is Great" and distributing sweets to passers-by, even as their leader, Yasser Arafat, said he was horrified.

As the US looked for a culprit, the name of world's most wanted terrorist, Osama Bin Laden, came to the fore.

In June, a US judge set today as the sentencing date for a Bin Laden associate for his role in the bombing of a US embassy in Tanzania that killed 213 people three years ago.