A Good Samaritan rescued a teenage boy who was repeatedly kicked in the head by two attackers as he lay on the ground, police said.
Scotland Yard said officers were searching for the unknown witness who intervened when he saw two men attacking the 16-year-old boy in Streatham.
The boy was knocked unconscious by his attackers who repeatedly kicked him in the head as he lay on the ground in Lewin Road, near the junction with Streatham High Road.
He was taken to hospital and discharged later that day.
Police were appealing for witnesses to the assault, which happened at about 7pm on December 13, and for any information that could help them to trace the man who stepped in to protect the victim.
DC Nicky Dixon of Lambeth CID said: "The witness who intervened certainly prevented an already nasty attack from escalating further and it is vital that I speak to him."
Police said there is no description available of the witness other than he is believed to be a white man.
If you believe you are the man who intervened to stop the assault or have other information that may assist the investigation, email DC Dixon at nicky.dixon@met.police.uk or call police in Lambeth via 101.
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