Cub scout leader Ingrid Loyau-Kennett selflessly engaged the terrorists in conversation and asked them to put down their weapons as one of them told her: "We want to start a war in London tonight."
Ingrid, 48, from Cornwall, was one of the first people on the scene after the two Islamic extremists butchered a soldier in south east London.
She was photographed by onlookers confronting one of the attackers who was holding a bloodied knife.
Ingrid was a passenger on a number 53 bus which was travelling past the scene, and jumped off to check the soldier's pulse.
Interviewed by newspaper and TV reporters, she said: "Being a cub leader I have my first aid so when I saw this guy on the floor I thought it was an accident then I saw the guy was dead and I could not feel any pulse.
"And then when I went up there was this black guy with a revolver and a kitchen knife, he had what looked like butcher's tools and he had a little axe, to cut the bones, and two large knives and he said 'move off the body'.
"So I thought 'OK, I don't know what is going on here' and he was covered with blood. I thought I had better start talking to him before he starts attacking somebody else. I thought these people usually have a message so I said 'what do you want?'
"I asked him if he did it and he said yes and I said why? And he said because he has killed Muslim people in Muslim countries, he said he was a British soldier and I said really and he said 'I killed him because he killed Muslims and I am fed up with people killing Muslims in Afghanistan they have nothing to do there."
Moments earlier, the killers had hacked at the soldier 'like a piece of meat', and when Ingrid arrived on the scene they were roaming John Wilson Street waiting for police to arrive so they could stage a final confrontation with them.
She said: "I started to talk to him and I started to notice more weapons and the guy behind him with more weapons as well. By then, people had started to gather around. So I thought OK, I should keep him talking to me before he noticed everything around him.
"He was not high, he was not on drugs, he was not an alcoholic or drunk, he was just distressed, upset. He was in full control of his decisions and ready to everything he wanted to do. I said 'right now it is only you versus many people, you are going to lose, what would you like to do?' and he said I would like to stay and fight."
The suspect in the black hat then went to speak to someone else and Ingrid tried to engage with the other man in the light coat.
She said: "The other one was much shyer and I went to him and I said 'well, what about you? Would you like to give me what you have in your hands?' I did not want to say weapons but I thought it was better having them aimed on one person like me rather than everybody there, children were starting to leave school as well."
Ingrid was not the only woman to show extraordinary courage. Others shielded the soldier's body as the killers stood over them.
MPs praised the 'extraordinary bravery' of the women and raised concerns about why it took armed police 20 minutes to arrive at the scene while people's lives were at risk.