Millions around the world watched on live TV as terrorists struck at the heart of the United States on September 11, 2001, killing thousands and leaving the most famous skyline in the world changed forever.

The United States was brought to a standstill as, within an hour, the most powerful symbols of US economic power had collapsed and 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, its apparent target Washington.

Almost 3,000 people were killed by the attacks seven years ago after suicide bombers seized four airliners.

Airports were closed, flights grounded, borders sealed, and jet fighters were scrambled above major cities, as the US tried to comprehend the enormity of what had happened.

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

The first strike was against the World 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.

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

Carnage

"Make no mistake," he said before being taken to the US Strategic Air Command headquarters in Nebraska. "The US will hunt down and punish those responsible for this cowardly act."

The scenes of unbelievable destruction were beamed live around the world on TV. 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 Rudy Giuliani took charge and urged people to be calm.

"I never thought I would see anything like this happen," he said. "I got there after the first plane hit and before the second. Watching people jump from the top of the World Trade Centre was an unbelievable sight."

Within hours, the name of Osama bin Laden emerged as the man responsible -- just three weeks earlier he had vowed to unleash an unprecedented attack against the US. He remains on the FBI's "most wanted list", with a reward of up to €18m for his capture.

- Wesley Johnson