A witness who came to the rescue of a three-year-old girl who was assaulted in the Clapham attack said she thought "she was going to die in my arms".
The woman - who wishes to remain anonymous - told the BBC she went to hospital with the child after seeing her "being slammed to the ground".
The girl, her sister, eight, and mother, 31, were attacked with a corrosive substance on Wednesday.
A nationwide manhunt is still ongoing for suspect Abdul Shokoor Ezedi, 35.
The mother is said to have experienced life-changing injuries, while her daughters suffered injuries not "as serious as first thought".
Speaking to the BBC, the witness said she saw Ezedi drag the younger girl from a car before slamming her into the ground twice - and running off.
"I heard a bit of shouting and I could hear it louder. So I ran outside. But I was just in shock. I first thought there had been a car crash or something," she said.
She continued: "The suspect slammed what I thought was a car seat or a bag to the floor after jumping from a white car.
"The sound, the thud, the scream, that's when I realised it was a little girl being slammed to the ground, like a WWF wrestling move.
"Then I saw two men jump on the suspect and try to pin him down, before he managed to get free and run off. So I just grabbed the three-year-old girl and took her into my arms before rushing her into the nearby hotel, where her older sister and mother were. Her mother was hysterical."
The resident - who said she was just about to settle down to watch EastEnders - went on to explain how she tried to reassure the three-year-old, who was barely conscious and whose head was "red and swollen".
It looked like the attacker was trying to "crack her head open", she said, adding: "It gives me shivers thinking about what he did."
She continued: "When I picked her up - the youngest child - she was trying to go to sleep. I knew from watching 24 Hours in A&E that the best thing to do for head injuries is to place somebody into the recovery position, so that's what I did.
"But before I did, she just went limp. I thought the youngest of the two girls was about to die in my arms. Luckily the three-year-old woke up and started to call for her mum.
"I took her to the waiting ambulance, she was clinging to me and didn't want to go, so I escorted her to hospital in the back of the ambulance, talking to her the entire way."
Later in the evening, the witness realised that she too had been splashed by the chemical. The BBC understands that 12 people in total were injured in Wednesday's attack.
"When I got home I noticed what looked like black paint on my foot. I went to shower it off but it wouldn't come off. Then I started to get a burning sensation," she said.
She went back to hospital and waited seven and a half hours for treatment, she said.
"My foot is now black and swollen. Because of the adrenalin of the whole event, I didn't think to check myself."
The witness added she had been left traumatised by the incident and wanted support after hearing the screams of the victims.
On Sunday, the Met Police said the last confirmed sighting of the suspect was at 21:33 on Wednesday, when he came out of Tower Hill Underground station.
He had arrived at Victoria on the Victoria line at 21:10 before taking an eastbound District line train to Tower Hill at 21:16.
Anyone with information is being asked to contact the police on 020 7175 2784 or for an immediate sighting dial 999.