This was very close to the nightmare the security services had dreaded.
By Nick Assinder
BBC News Online Political Correspondent
Thankfully it was a harmless protest. But it could have been much, much worse.
Tony Blair hit by the powder
And it took some time before the full impact of what had just happened in front of our eyes sank in.
A couple of men somewhere in the so-called "posh seats" of the public gallery - where only Lords, dignitaries and those vouched for by MPs can sit - lobbed three bags or balloons of purple powder at the prime minister.
One appeared to land near his feet, another hit him squarely on his shoulder before bursting and then scattering around the feet of Chancellor Gordon Brown.
MPs immediately behind the PM started waving their order papers to clear the clouds of the purple powder hanging in the air - or brushed it off their clothes.
There was no immediate sign they were overly bothered. And we all just stared at the public gallery as the men were bundled out by Commons officers.
The prime minister appeared unsure of what was happening to him before he was ushered slowly, almost bemused, from the chamber.
John Prescott, on his right, was still laughing at Michael Howard's last joke - something about him and Gordon Brown having just stitched up the PM like a kipper.
Gordon Brown was as inscrutable as ever. Maybe he hadn't seen the projectile.
Then it started to hit home and the faces changed. Officers and party whips started shouting at the MPs to get out of the chamber.
Mr Prescott, now looking grim, stayed behind to place pieces of paper over the powder on the floor, presumably to stop it scattering any further.
Everyone attempted not to betray their nervousness, but it was on everyone's mind.
Blair was hit by powder
Was this possibly the chemical terror attack the security services had feared and even warned about?
It was those warnings that led to the erection of the controversial security screen in front of the public gallery. A screen that failed to stop this stunt.
But there was no panic, even as the full realisation of what had just happened sunk in.
As MPs filed out of the chamber there was a mixture of irritation, anger and just a little nervousness.
"What was it" and "who were they" were the questions on everyone's lips.
It soon emerged it was a "harmless" demonstration by Fathers' for Justice. That both infuriated and relieved MPs.
It infuriated because of the "irresponsibility" of the action and it relieved because it was not something more sinister. But perhaps it might have been.
One MP said he had great sympathy for the group's case. "But they have just done themselves a huge disservice," he said.
Others expressed dismay that the event would remove any arguments against the security services' attempts to impose draconian measures on the Commons.
"The security services already wanted much tougher security - we headed off the worst but they will get their way now," said another.
None would indulge too much in the "what ifs", but it was clearly on their minds.
"Maybe they've done us a service. The new security measures didn't work," said one.
Back to normal
Then the word went out that it was a Labour peer who had signed in the protesters.
Her actions attracted some sympathy mixed with groans of disbelief and mutterings of "there but for the grace of God".
Speaker Michael Martin, who had taken control of the situation instantly and with great authority, soon declared it was "back to normal".
And MPs filed back into the Chamber to show they were not about to be intimidated.
But, of course, it was not back to normal. It probably never will be quite the same.
The consequences of this single act will engulf Westminster.
Many MPs previously needed persuading that the security screen was not an affront to democracy.
If the security services had been given their head, security arrangements would have been many times more severe - one paper suggested a concrete wall around the Commons had been one suggestion.
That might not happen but those other ideas will now come back and it is hard to see how they can be resisted.
Thanks to Fathers 4 Justice, Parliament is about to become a fortress.