Abdullah Tanoli was working as a security guard at a shop on the busy central London plaza when he heard the girl's terrified screams.
"You know when a child is just crying, but when someone is being hurt, that scream is different," the 30-year-old told the BBC.
The girl told police she thought she was going to die after she was targeted by Ioan Pintaru on the morning of August 12, 1994 while on holiday with her mother.
Pintaru, 33, approached the girl as she left the Lego store where she and her mother had been buying gifts.
He placed her in a headlock and stabbed her eight times in the face, neck and chest.
"I never thought I would see something like that," Tanoli told the BBC.
"At that moment, the only thing in my mind was to save the child."
He managed to grab Pintaru's hand holding the knife, which caused the weapon to drop and was able to kick it away.
The guard and two other men pinned Pintaru down before police arrived minutes later and arrested him.
A nurse walking past help stem the girl's bleeding.
Pintaru was sentenced at the Old Bailey on Tuesday to a hospital order under Section 37 of the Mental Health Act and a restriction order under Section 41 - meaning he can be detained indefinitely.
During sentencing, Judge Richard Marks KC commended Tanoli for his bravery and ordered a reward of Stg1000 ($A2000).
The girl's mother contacted Tanoli on social media a week after the attack and they have remained in contact.
Tanoli said of the girl, who can't be named for legal reasons: "I consider her like my younger sister".
"If you see her now, she looks normal. I am very relieved."
The court heard the child, now 13, has recovered physically from her wounds but "invisible scars" remain.
"The psychological effects of this incident will remain with (her) for the rest of her life," prosecutor Heidi Stonecliffe told the Old Bailey.
Detective Constable Laura Nicoll, of the Met's specialist crime team, who led the investigation, said "it was a savage and brutal attack that left the city in shock".
"But ... we also saw the best of London: a security guard who raced from his post without a thought of his own safety, the off-duty nurse who tended to the little girl's wounds," she said.
"Without them, she might not be alive today."
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