The 17-year-old Kurdish Iranian was waiting at a bus stop with two friends in Croydon, south London, when he was set upon by about eight youths.
They asked him where he was from before chasing him down the road and repeatedly kicking him on the ground.
The Met Police said the boy sustained serious head injuries.
His two friends escaped with minor injuries.
The attack happened in Shrublands Avenue at 23:40 BST on Friday. No arrests have been made.
Croydon's Metropolitan Police Borough Commander, Ch Supt Jeff Boothe, said the boy was the victim of "a frenzied attack by a large number of people".
He said the victim was kicked repeatedly while on the floor and "by all accounts with members of the public asking them [his attackers] to stop".
Ch Supt Boothe added it was only the sound of police sirens which stopped "this horrendous and frenzied attack".
Earlier Det Sgt Kris Blamires said: "At this early stage it is believed that about eight suspects approached the victim as he waited at a bus stop with two friends outside The Goat public house in the Shrublands.
"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 critical head and facial injuries as a result of this attack, which included repeated blows to the head by a large group of attackers."
A number of people came to the aid of the victim as he lay unconscious and injured following the assault, the Met said.
The police are urging those who helped the boy and anyone who witnessed the attack to get in touch
A young asylum seeker is in a serious but stable condition after a "frenzied attack" by a gang, which police are treating as a hate crime.