Police want the public to help trace a man who masturbated on a train between London Waterloo East and Orpington.
The incident happened shortly after 1pm last Wednesday (April 9) as a 43-year-old woman travelled towards Petts Wood station.
During the journey the woman noticed a man staring intently at her, making her feel extremely uncomfortable.
Police Constable Michael Willoughby said: "The woman was frightened by the man and moved into another carriage, but he followed her and sat next to her.
"She then noticed he had his hand down the front of his trousers and was performing an indecent act on himself – all the time staring at the woman.
"She moved again, but was again followed by the man."
Officers are in the early stages of investigating the incident and are in the process of gathering all potential evidence.
They hope other passengers may have information which could assist the investigation.
PC Willoughby added: "The offender was a black man, aged about 50, was about 5’10", of large build and had short hair which was almost shaved bald.
"At the time of the incident he was wearing a dark t-shirt, a navy cotton top and dark grey cotton jogging bottoms.
"It is imperative that we trace this man as soon as possible as his behaviour was completely unacceptable and is exactly what we are trying to stamp out through Project Guardian."
If you have any information about the incident please call BTP on 0800 40 50 40, text 61016, or ring Crimestoppers, anonymously, on 0800 555 111. In all calls please quote log SSUB/B10 of 16/4/14.