CCTV footage of the incident shows the man taking an enormous swing at the girl with his right fist before contact.
The video then shows the man running off down the street leaving his stricken victim unconscious on the pavement.
The victim was taken to Newham General Hospital, she suffered bruising and cuts to the face, but was well enough to be released later that day.
Police urge anyone with any information as to the identity of the attacker, described police as a black male, with a shaven head, wearing a baseball jacket and jeans, to come forward.
Call the Newham Violent Crime Unit on 020 8217 5890