A 10-year-old boy was sexually assaulted in front of his mother at Leicester Square Tube station in central London.
The schoolboy was walking with his mum through the busy underground station when a man groped him as he walked up the stairs, British Transport Police (BTP) said.
His mother chased after the suspect as he fled but she could not catch him, the force added.
BTP have released a CCTV image of a man they would like to speak to over the incident which happened at around 4.30pm on 16 February.
A spokesman for the force said on Friday: "Do you recognise this man? Officers investigating a sexual assault at Leicester Square Underground station are releasing this image in connection.
"At around 4.30pm on Thursday February 16, a 10-year-old boy was walking up a set of stairs at the station with his family when a man sexually assaulted him and ran away.
"The victim’s mother chased the man through the station, but didn’t catch him.
"Officers believe the man in the CCTV image may have information which could help their investigation. If you recognise him, or have any information, please contact BTP."
Anyone with information can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 474 of 16 February, or call Crimestoppers anonymously on 0800 555 111.