The 36-year-old, originally from the Dominican Republic, was buying a mop and bucket for her new flat when Amman — who was being watched by armed police — entered the high street shop and made to steal the knife.
“But when he reached the door he pushed me,” Anna, which is not her real name, told the BBC. “He tried to open and remove the plastic packaging from the knife but he didn’t manage it.
“He pushed and he stabbed me but the knife was still covered with plastic.”
Then, Anna recounted that “pushing everything that was in his path as he ran away”, Amman then stabbed 51-year-old nursery school teacher Monika Luftner as she took the lock from her bike, before he ran up the High Road “like a mad person and throwing his knife around”.
The Metropolitan Police has said the shopkeeper chased Amman up the street as the 20-year-old attacked Ms Luftner and a man in his forties before being shot dead by armed police.
“I spent 15 to 20 minutes in hell,” Anna said in Spanish, describing the attack as “very quick, like in a movie.
Anna told the broadcaster she had not been able to sleep since the Sunday attack, adding: “I have to go to work and walk around the streets ... this is something that stays with you. It’s a really bad trauma.
“They might have had to call my mum and my daughter to let them know I was dead.”
Amman, 20, had been automatically released from prison days earlier, half-way through a sentence for collecting and distributing material useful for terror attacks, and was living in a Streatham probation hostel.
Undercover officers, who deemed Amman dangerous enough to warrant carrying arms, had been trailing him on foot for 40 minutes when he entered the shop. He was wearing a suicide vest “concealed under his clothes”, according to Scotland Yard.
Police monitoring is believed to have intensified in the days before the attack as his behaviour grew more disturbing, and Metropolitan Police commissioner Dame Cressida Dick was questioned by the London Assembly on Wednesday over whether the attack could have been prevented.
“They are conducting covert surveillance, so they are not of course providing man-to-man marking,” Dame Cressida said. “They are there covertly and that is a deliberate thing. It is inevitable that there could be a time delay before somebody totally unexpectedly does something.”
She added: “I wish I could assure the public that everybody who poses a risk on the streets could be subject to some sort of thing that would stop them being able to stab anybody ever, but it is clearly not possible.”
Scotland Yard said Amman was inside the shop for less than a minute, and that he was shot dead within 60 seconds of starting the attack.
The man in his forties stabbed by Amman remains in a “serious, but stable” condition, Scotland Yard said on Wednesday, while a woman in her twenties who was wounded by shattered glass caused by gunfire had been discharged from hospital by Monday night.
Ms Luftner was discharged from hospital on Sunday and is recovering at home with her partner. St Bede’s Catholic Infant and Nursery School said a staff member was making “making a good recovery after experiencing a shocking attack” and asked that her privacy be respected.
The head of UK counterterror police, Neil Basu, renewed calls for people to report any suspicious behaviour, writing in the Evening Standard: “The threat is, despite our best efforts, not diminishing.
“With 3,000 or so subjects of interest currently on our radar and many convicted terrorists soon due to be released from prison, we simply cannot watch all of them, all the time.”
The government has vowed to introduce emergency laws to stop the early release of terror offenders currently in prison and ensure they are assessed by the Parole Board. It has not ruled out effectively suspending the European Convention on Human Rights in order to apply the new measures.
But ministers have warned the plans could see extremists merely serving their full sentence in prison before being released without conditions restricting their movement, communications and association — which they are currently subject to when released on licence mid-way through their sentence.
Figures suggest some 50 terror offenders could be released from British prisons this year, with 224 inside at the most recent count.
But the number of flagged extremists in British jails is far higher, comprising at least 800 offenders.
Despite Amman’s terror conviction, his mother, Haleema Faraz Khan, told Sky News she believed her son had been radicalised inside Belmarsh high security prison, where concerns have previously been raised over Islamist extremism and networking between terrorist prisoners.
Additional reporting by PA