A boy is in a critical condition in hospital after four people were stabbed in south London.
The victims, aged between 15 and 16, were attacked outside Landor House on the Elmington Estate in Camberwell on Thursday evening, police said.
One remains critical, another is in a serious but stable condition and the other two were not seriously injured.
Six boys, aged between 15 and 16, have been arrested on suspicion of violent disorder and grievous bodily harm.
An eyewitness described seeing one boy with severe wounds to his abdomen. He was seen running around the corner of the building before dropping to the ground.
The four victims were later taken to hospitals in south London for treatment.
A Section 60, granting police stop and search powers across the area, was authorised until 09:30 on Friday.
Supt Annmarie Cowley said the Met Police’s investigation was “in its very early stages and at present we cannot speculate on what the nature of it may be”.
She described the attack as “beyond comprehension” and said the order had been granted “to deter any further potential trouble”.
The arrested teenagers remain in police custody.
At the scene: Ross Ryan, BBC Radio London
There’s very little to suggest something serious has happened here.
A police officer told me forensic experts had been and gone so everything’s been cleared away apart from a cordon which still runs right the way round the building.
There’s several officers in the area now just to guard the crime scene.
It’s only two weeks since I was in the area reporting on the death of Sidique Kamara, just 10 minutes’ walk away.
Witnesses said they heard a teenager shouting “Help, help” and saw a group of boys running minutes before police arrived.
Officers were called shortly before 17:25 BST.
A neighbour described seeing police giving first aid to one boy as he lay on the ground.
Another woman, who did not want to be identified, said what she saw was “very traumatising”.
Harriet Harman, Labour MP for Camberwell and Peckham, said this was the third violent attack in her constituency since May.
Rhyhiem Ainsworth Barton was found with fatal gunshot wounds on Warham Street in May, while his friend Sidique Kamara was stabbed to death on the same street on 1 August.
Ms Harman told BBC Radio London: “It’s absolutely dismaying that this has happened again.
“Not only is this really a tragedy for the families, when young men, so young, are involved in this sort of crime… But it creates real shock in the local community.”
Meanwhile, a murder investigation is under way after a man was stabbed to death in Chingford, north-east London.
Police found the 42-year-old victim with stab wounds after they were called to an address in Flaxen Road, Waltham Forest, at about 04:40.
He was pronounced dead at the scene at 05:22. A 16-year-old boy and a 35-year-old woman have been arrested on suspicion of murder.