The attack left the 17-year-old Iranian Kurd in serious but stable condition in a London hospital. Authorities are treating the matter as a suspected hate crime.
Detective Sergeant Kris Blamires said the teenager was at a bus stop with two friends when approached by about eight youths.
"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," Blamires said in a statement.
The teen's friends managed to get away.
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British police say hate crimes remain under-reported in Britain. The country has seen a surge in xenophobia expressed in threats, taunts and physical attacks after Britain's vote to leave the European Union last year.
Much of the Brexit debate focused on an influx of migrants into Britain from other EU countries and whether their presence was making it harder for Britons to find work, housing and medical care.
London's police force has increased the number of specialist investigators by 30 percent in the past two years, with more than 900 dedicated to investigating such attacks.