A cub scout leader confronted terrorists just seconds after they had beheaded a soldier asking them to hand over their weapons and warning them: "It is only you versus many people, you are going to lose."
Ingrid Loyau-Kennett, 48, was on a bus heading through Woolwich in south east London yesterday afternoon when she spotted the stricken soldier lying bloodied in the road.
Her bravery - and that of others who tried to reason with the killers - has been praised, particularly in the wake of amateur footage from the scene, which shows one of the two suspects making political statements about the slaughter while still brandishing weapons.
Although the identities of the men have yet to be revealed, both of the suspects are British nationals and one of them is of Nigerian descent.
Cub leader Ms Loyau-Kennett was speaking today ahead of a meeting by the Government's emergency response committee, Cobra, which is due to discuss security measures following the attack.
Ms Loyau-Kennett, of Helston in Cornwall, said she initially thought the victim had been injured in a car crash after spotting a badly damaged vehicle on a pavement at the scene.
She said: "I went to the guy and when I approached the body there was a lady cradling him. And then (one of the killers), the most excited one of the two, said, 'Don't go too close to the body'.
"I thought, okay. And because I was down I could see a butcher's knife and an axe - that's what he had - and blood. I thought, what the heck? I thought obviously he was a bit excited and the thing was just to talk to him."
Ms Loyau-Kennett said she tried to reason with the killer in an effort to focus his attention away from other potential victims, as large crowds began to huddle at the scene.
She said: "I know it's big today but for me it was just a regular guy, just a bit upset. He was not on drugs, he was not drunk.
"He said, 'Don't touch, I killed him'. I said, 'Why?' He said: 'He's a British soldier. He killed people. He killed Muslim people in Muslim countries.'
"And I said: okay. So what would you like? I tried to maker him talk about how he felt. He said all the bombs dropping and blindly killing women, children...
"More and more people were starting to come. There were so many people around. I just looked around and I found it so daunting."
However, Ms Loyau-Kennett said her thoughts were to "just carry on" talking to the man, while several woman arriving at the scene tried to shield the victim.
She said: "I wanted him to concentrate on me and make sure he doesn't have a funny idea.
"He (the killer) told me he was a British soldier - he didn't look like a British soldier to me, he wasn't in uniform. But I thought if another one passes by, or is in the area..."
Asked if she was scared, the woman replied: "No - better me than a child.
"Unfortunately there were more and more mothers with children stopping around, so it was even more important I was talking to him and ask him what he wanted."
Woolwich and Greenwich MP Nick Raynsford today praised the "extraordinary" bravery of members of the public who approached the killers. The men are currently in separate London hospitals being treated for injuries after they were shot by police at the scene.
Relatives of the dead soldier are believed to have been informed though he has yet to be named.
One of the attackers behind the barbaric killing was filmed wielding a bloodied meat cleaver, saying: "We swear by almighty Allah we will never stop fighting you."
In the chilling footage, he explains his terrifying actions.
"We must fight them as they fight us. An eye for an eye, a tooth for a tooth," he is heard to say in the clip, obtained by ITV News.
"I apologise that women have had to witness this today, but in our land our women have to see the same. You people will never be safe. Remove your government, they don't care about you."
The attacker, who spoke clear English without a foreign accent, is then seen walking towards the victim, who is lying in the street. Another man is standing by the damaged car.
The incident occurred some 200 yards from the Royal Artillery Barracks, adjacent to Woolwich Common, the historical home of the Royal Artillery.
The barracks, also known as the Woolwich station, now houses a number of the King's Troop Royal Horse Artillery and independent companies of the Grenadier and Coldstream Guards.
A car believed to have been used in the attack was taken away during the night.
The blue vehicle, which appeared to have collided with a road sign in John Wilson Street, was covered with a red tarpaulin and taken away by a tow truck.
Former chairman of the Joint Intelligence Committee Baroness Neville-Jones said the security services would today be exploring whether the attack was carried out by a "lone wolf" or by someone with further connections, either at home or abroad.
"Clearly, as in this case the perpetrators are still alive they are going to be questioned. There is going to be a great deal of information available," she said on the BBC Radio 4 Today programme.
"There is a much bigger problem, potentially.
"Isolated attacks of the kind we have just seen, of this kind of attack, I'm inclined to think is possibly more in the nature of a lone wolf, is particularly hard to deal with because there are very few outward signs beforehand of the nature the intelligence services can pick up."
Baroness Neville-Jones welcomed the moves by Muslim community leaders to condemn the attacks but warned more action needed to be taken to ensure radical rhetoric was properly tackled and "talking to people about what the proper nature of democracy is".
Colonel Richard Kemp, who commanded British forces in Afghanistan, said he did not believe reinstating a bar on military personnel wearing uniforms in public would be the right approach.
Speaking on the same programme, he said: "Personally, I would argue against it. As we saw in this case you don't need to have somebody in uniform, you just need to have someone who knows a bit about soldiers and does a bit of observation in the vicinity of a barracks and you can identify a soldier very quickly.
"I think we should be right to think about ways of protecting ourselves better but I think it would be wrong to suggest we live in a state of fear of this type of attack continuing.
"I think it is possible further attacks will be inspired by this type of attack... one of the biggest priorities for the services is to look at the role of the internet in motivating people and look very carefully at which radical sites should be suppressed on the internet, as well, of course, as more direct preaching in some of the mosques in this country which has caused people to turn to radicalism and terrorism before.
"That's another area we need to put more resources into again."
Mr Johnson said: "The fault lies wholly and exclusively in the warped and deluded mindset of the people who did it.
"What we need to do now, for the sake of the victim and the sake of his family, is for these killers to be brought to justice."
Mr Johnson paid tribute to the people of Woolwich for showing "such exemplary courage".
He went on: "People should take their cue from the behaviour of people of Woolwich who showed such natural courage and stood up to those killers. That's the spirit of London."
The mayor urged London's citizens to "go about their lives in the normal way".
In a message on Twitter, Mr Cameron said: "I have been updated by the commissioner and will chair Cobra shortly. I will make a statement on this sickening killing this morning."