The teenager was crossing Doris Road, Forest Gate, around 3pm when a green-coloured car stopped sharply.
After an exchange of words between the boy and the driver, four or five Asian males got out of the car.
One of the men removed a metal pole from the boot and hit the boy across the head, leaving a gash wound and knocking a tooth out, before driving off towards Upton Lane.
The boy was discharged from hospital that day.
A 23-year-old man was arrested for grievous bodily harm on Friday and bailed to a date in late April.
A police spokesman said the attack appears to be “unprovoked” and asks witnesses to call detectives at the Newham Violent Crime Unit on 020 8217 8990 or call Crimestoppers anonymously on 0800 555 111.