A man was seen "hiding in the bushes" and "masturbating" near a football stadium late at night.
Police were called to Winfield Way, outside The People's Pension Stadium, in Crawley shortly before 11pm on Sunday following reports of a man indecently exposing himself.
The man was spotted by a dog-walker, who reported him to police. The man was later seen making his way towards the town centre.
Officers attended and searched the area, but the culprit could not be tracked down.
Sussex Police are now urging anyone who saw any suspicious activity in the area at the time of the incident to contact them as enquiries continue.
A force spokeswoman said: "Police were called at about 10.45pm on Sunday, February 28, to a report of a man indecently exposing himself in Crawley.
"The man had first been seen in Winfield Way heading towards Crawley Avenue, and then again in Brighton Road. He left in the direction of the town centre.
"The man is described as being in his late 20s or early 30s with black hair. He was wearing a jacket over a purple and orange t-shirt, and black trousers.
"Officers attended and carried out a search of the area but found no trace of the man. Enquiries are ongoing and anyone who saw any suspicious behaviour at that time, or who may have any relevant CCTV, doorbell or dash cam footage from the area, is asked to contact police online or by calling 101 quoting serial 1387 of 28/02."
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