Police are investigating a robbery that took place in an alleyway between Kenway and East Street, Southend, at around 3pm on Friday, August 11.
A teenage boy was approached by three men, before one of them pulled out a knife. They stole his phone and bag and left towards East Street when passers-by approached.
The men are described as being white and aged between 13 and 17 years old. They were wearing hooded tops and bandannas covering their faces.
Officers are particularly keen to speak to the two people who walked by.
Witnesses or anyone with information is asked to contact Southend CID on 101 or Crimestoppers anonymously on 0800 555 111.