The teenager was badly beaten by a group of around 20 people while he was at a bus stop with two friends outside a pub in Croydon on Friday night.
"It is understood that the suspects asked the victim where he was form and when they established that he was an asylum-seeker, they chased him and launched a brutal attack," police investigator Gary Castle said.
"He 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," he said.
Police said the teenager's condition was "serious but stable and is not believed to be life-threatening."
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The attack has been widely condemned and local MP Gavin Barwell has described the attackers as "scum".
London Mayor Sadiq Khan said hate crime "has no place in London, Britain or anywhere else" and the Refugee Council charity said it was "disgusted" by the attack.'
The Daily Mail tabloid called the assailants "SAVAGES" in a front-page headline and compared the attack to the racially motivated murder of Stephen Lawrence, a black teenager set upon by a gang of white youths in 1993 in another part of south London.
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