A 16-year-old boy was hit several times over the head with a claw hammer before getting on to a bus to avoid being assaulted further, police said.
Scotland Yard said the "horrific" attack nearly left the school pupil unconscious due to blood loss after he was confronted at a bus stop on Poplar Park, East India Dock Road, in Tower Hamlets.
The victim, who needed five stitches in the back of his head, was able to get on to a bus while passengers prevented the assailant from boarding.
Officers were called at around 4.10pm on October 4 and the suspect - a black teenager, wearing dark trousers and a top - ran in the direction of Burdett Road.
PC Jon Privett from Tower Hamlets borough said: "This is a horrific attack on a school boy. The boy nearly lost consciousness from blood loss as a result.
"The quick action of the passengers and bus driver prevented the boy from further attack, and they were able to contact emergency services promptly."
Anyone with information is asked to call police on 101 or Crimestoppers anonymously on 0800 555111.