The 17-year-old, a Kurdish Iranian, is fighting for his life in hospital after being targeted by around eight people in what detectives are treating as a hate crime.
The boy was waiting at a bus stop with two friends outside The Goat pub in Croydon, south London, when the group approached them.
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.”
Six people have been arrested on suspicion of attempted murder, four men and two women, all aged between 20 and 24, are being held at a south London police station for questioning. They were arrested at addresses in Croydon late on Saturday on suspicion of attempted murder and violent disorder.