Investigators believe the unknown man was seen masturbating while looking at a mother and her child.
The incident unfolded near Murray Road, in the vicinity of the Preston Railway Station, just after 2pm on February 28.
Police have released CCTV and images of a man they believe can assist with their enquiries.
The man is perceived to be of Middle Eastern appearance, approximately 20 to 25-years-old, with a solid build, short dark hair and olive skin.
He was wearing black pants, a white t-shirt and carrying a black backpack.
