A teenager cried for help before four young men were stabbed on a south London housing estate.
The victims are thought to be teenagers.
Residents of the Elmington Estate in Camberwell have said a group of boys running minutes before police arrived on the scene.
The incident took place shortly before 5.25pm on Thursday.
Scotland Yard has said six males were arrested and are being held in custody.
Their condition is not known but witnesses who saw the aftermath described at least one teenager who appeared to be seriously hurt.
“That particular one, I hope he does live but by the looks of it and the way the ambulance had to struggle I would give it just an 8% (chance),” said a 25-year-old woman, who did not want to be identified.
“It was very traumatising.”
The crime scene – littered with bloodied clothing, trainers and medical equipment – is just a short walk from Warham Street, where 23-year-old drill rapper Sidique Kamara, also known as Incognito, was stabbed to death on August 1.
Another neighbour, who did not give her name, described seeing boys running on the grass opposite her flat shortly before the stabbing.
“There were four or five black boys running. All of a sudden I could hear, ‘Help, help’,” she said.
“I have walked up and as I did I have seen the police running.
“One of the police officers got a first aid bag and I saw one of the boys lying on the floor.”
Harriet Harman, MP for Camberwell and Peckham, said she was very concerned by the quadruple stabbing and is in contact with police and councillors.
Superintendent Annmarie Cowley said: “This incident is in its very early stages and at present we cannot speculate on what the nature of it may be.
“What we can say for certain is if there needed to be an example of the utter senselessness of knife crime, then this is it.
“Four males are in hospital, and at this stage we cannot say how serious their injuries are. It is beyond comprehension.”
She added that legislation has been invoked to grant officers powers to deter further trouble overnight.