Up to 20 people watched on as a gang punched and kicked a young asylum seeker so badly he was left with a fractured skull and blood clot on the brain, neighbours say.
Nine people have been arrested on suspicion of attempted murder after a "savage attack" on a suburban street left the 17-year-old fighting for his life.
The teenager, a Kurdish Iranian, was waiting at a bus stop with two friends outside The Goat pub in Croydon, at 11.40pm on Friday when the group approached them.
They chased him and launched a vicious assault, repeatedly kicking him on the floor and aiming blows to his head.
Police believe a gang of around eight were involved in the attack but residents said many more watched on while the boy was beaten on the ground.
One resident of Shrublands Avenue, where the attack took place, said: "There were more than eight people, there was a massive group coming up from The Goat pub.
"You couldn't see who was hitting who.
"There was one person, the one who ended up in hospital, he was getting absolutely beaten up - kicked, mostly in the facial area. He was getting kicked and punched.
"There was a group of roughly 10 people that was kicking and punching him, and the rest, another 10 or 20, were all just around watching.
"Then when they heard sirens they started to go. People that were there, witnesses, because they couldn't do anything against a group of 30 people, they had to wait until they moved off and that's when they managed to help him.
"That's eventually when the ambulance and police (arrived), and that was it."
The boy remains in hospital in a serious but stable condition, Scotland Yard said, while officers have been in the process of contacting his next of kin.
Three 20-year-old men, a 20-year-old woman and a man and woman both aged 24 were arrested at addresses in Croydon late on Saturday on suspicion of attempted murder and violent disorder.
A 23-year-old man, a 26-year-old man and a 17-year-old girl were all arrested on suspicion of attempted murder on Sunday after handing themselves in at south London Police stations.
Detectives are investigating whether some of those involved had been drinking in The Goat pub nearby.
Managers at the pub said they were "saddened" to hear of the incident, adding: "We do not in any way condone such action and we will do anything within our powers to help police to bring those that carried out this act to book. To the victim, we pray for his speedy recovery."
Chief Superintendent Jeff Boothe, Croydon's borough commander, said the "close-knit community" has been left shocked by the ferocity of the attack, he added, and extra officers have been out and about to reassure people.
He said: "This is not usual for the area, it is out of the norm. This is not Croydon, Croydon is a very diverse community - they celebrate their diversity."
Mr Boothe added: "Hate crime is something which we understand can be very, very divisive. Croydon is culturally diverse and we need to continue to celebrate that."
Gavin Barwell, Croydon Central's MP, said: "It's an appalling crime and I hope the people responsible are caught quickly and receive the full force of British justice."
Anyone with information about the attack should call police on 101 or Crimestoppers on 0800 555 111.
Additional reporting by the Press Association.