Mr Blair appeared shaken by news of the attacks
It took time for the full impact of the atrocities to hit home.
Union delegates at the TUC conference in Brighton were waiting for Tony Blair's keynote speech when the whisper started going around the hall.
At first, few believed the claim that terrorists had bombed the world trade centre in New York.
And the claims became increasingly extravagant and unbelievable.
Suicide squads
Two passenger jets had been hijacked by suicide squads who had then plunged them into the twin towers of the centre.
Then it was claimed that the Pentagon - the very heart of the US military complex - had been hit.
The first instinct was to laugh it off as an elaborate hoax or a malicious exaggeration until, thanks to instant news coverage, the unthinkable was made concrete.
This was probably the worst act of terrorism in recent history and countless lives had almost certainly been lost.
And as the awful reality began to hit home, the next emotion was shock, tinged with fear.
This was an act of war with potential consequences no one wanted to contemplate.
Tony Blair was quickly briefed on the horror and made an instant, instinctive and inevitable decision to cancel his speech.
He was then rushed back to London to chair a crisis cabinet committee meeting - with the inevitable fears that the UK could be the next target.
Almost immediately the TUC conference was also suspended, possibly indefinitely.
But there was still a sense of disbelief over what was unfolding on the numerous TV monitors around the Brighton conference centre.
As pictures of one of the trade centre towers collapsing were beamed into the centre the horror sunk in.
"If they can do this what might they do next," was the question on many lips.
Past bombs
Memories of the Grand Hotel bombing in 1984, the Oklahoma city bombing and numerous IRA attacks were immediately recalled.
But this was of a different order.
In a matter of minutes, the world had become a terrifyingly vulnerable place.