A 14-year-old girl was knocked off her bike in an underpass during an incident in Shenley Brook End.
At about 4pm on Wednesday, the victim was on her bike in an underpass between Shenley Brook End and Loughton, at the end of Lowndes Grove, when she was knocked off by a woman.
The woman grabbed the girl and shouted at her before a member of the public got involved and she left the scene.
Police would like to speak to the woman who intervened, as she could hold vital information for the inquiry.
The victim suffered a grazed knee during the incident.
Any witnesses can call police on 101 or Crimestoppers, on 0800 555 111.