Witnesses say up to 20 people watched as a 17-year-old asylum seeker was kicked and punched in a mob assault in south London on Friday night.
"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."
The teenager remains in hospital in a serious but stable condition which is not believed to be life-threatening, police said.
Officers said they had made nine arrests with eight remaining in custody and have also released pictures of three more people they wish to identify.
"Whilst arrests have been made, we now know that a large number of people, approximately 20, were involved in the attack on the victim," Castle said.
In a tweet after the attack local MP Gavin Barwell described those responsible as "scum" while London Mayor Sadiq Khan vowed the perpetrators would be brought to justice.
"London is an amazingly diverse city. We don't just accept our differences, but we embrace and celebrate them," Khan said in a statement.
"Our communities will not be divided by those who seek to sow hate. And we will always take a zero tolerance approach to hate crimes of any type."
Britain has seen a surge in xenophobia expressed in threats, taunts and attacks after British voters chose to leave the European Union.
