Six people have been arrested on suspicion of attempted murder over a "savage" attack that left a teenage asylum seeker with a fractured skull and a blood clot on the brain.
Four men and two women in their 20s are being held at a south London police station for questioning, Scotland Yard said.
Neighbours claimed a group of up to 20 people watched on as the young man was punched and kicked.
Government minister Gavin Barwell condemned the "appalling" attack that police are treating as a suspected hate crime.
The 17-year-old victim, a Kurdish Iranian, was waiting at a bus stop with two friends outside The Goat pub in Croydon, south-east London, when the group approached them.
After discovering he was an asylum seeker they chased him and launched a vicious assault, repeatedly kicking him on the floor and aiming blows to his head.
The teenager suffered serious head and facial injuries, including a fractured skull and a blood clot on his brain, and remains in hospital in a serious but stable condition.
His two friends escaped with minor injuries, Scotland Yard said, as officers were in the process of contacting the teenager's next of kin.
When they established that he was an asylum seeker they chased him and launched a brutal attack
Det Sgt Kris Blamires
Police believe he was approached by about eight suspects shortly before the attack in Shrublands Avenue at 11.40pm on Friday.
Detective Sergeant Kris Blamires from Croydon CID said: "It is understood that the suspects asked the victim where he was from and when they established that he was an asylum seeker they chased him and launched a brutal attack.
"He has sustained serious head and facial injuries as a result of this attack, which included repeated blows to the head by a large group of attackers."
Chief Superintendent Jeff Boothe, Croydon's borough commander, called it a "savage attack" and said it was only the intervention of passers-by and the arrival of police that stopped it being worse.
He said: "A number of bystanders and eyewitnesses tried to intervene and say to the attackers that enough is enough.
"By all accounts they didn't actually stop until the sound of police sirens were heard in the background."
The Refugee Council said it was "disgusted" by the assault.
Director of advocacy Dr Lisa Doyle said: "We hope the perpetrators of this appalling attack are swiftly brought to justice and we wish the victim a full and speedy recovery."
Mayor of London Sadiq Khan also condemned the attack:
Hate crime has no place in London, Britain or anywhere else. The perpetrators of Friday's attack in Croydon will be brought to justice. pic.twitter.com/qxNa0KiCSQ— Sadiq Khan (@SadiqKhan) April 2, 2017
Detectives are investigating whether some of those involved had been drinking in The Goat, Mr Boothe 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."
He 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.
"We are appealing to all decent people from whatever background they come from to help us identify the individuals that are involved in this isolated attack."
Mr Boothe said it was "only going to be a matter of time" before all the attackers were arrested.
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.
The attack happened outside The Goat pub in Croydon, south London
Housing minister Mr Barwell, MP for Croydon Central, said: "I think most people in Croydon will be as appalled as I am that what appears to have happened is a young man who came to this country seeking sanctuary has apparently been targeted because of his ethnic background.
"It's an appalling crime and I hope the people responsible are caught quickly and receive the full force of British justice."
Shadow home secretary Diane Abbott suggested the Conservatives had failed to get to grips with hate crime.
She said: "Sadly this is not an isolated incident but part of a sustained increase in hate crimes that this Tory Government is yet to offer any effective response to."