A man exposed himself to a dog-walker near a children's playground in Newbury, according to Thames Valley Police.
He followed the victim, a woman in her forties, before committing the offence at around midnight in Dene Way as it crosses the River Lambourn into the Shaw Park area.
A play area is located just 80 yards away from the bridge.
The offender is described as a white man, of slim build, and is around 5ft 9ins tall and described as being late teens to early twenties.
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Investigating officer, Detective Sergeant Darren Little, said: “We are appealing for anyone who may have seen what happened or saw someone matching this description acting suspiciously in the area, to please come forward.
“I am particularly keen to speak with a man who walked past the victim after the offender ran off and may have passed the offender.
“Anyone with information can call 101 or make a report through the website, quoting reference 43220289511.
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“Alternatively, if you wish to remain anonymous, you can contact the independent charity Crimestoppers on 0800 555 111 or via its website.
“We take reports of this nature extremely seriously.
“I would encourage anyone who has been a victim of this type of crime to report it to us and we will investigate thoroughly.”
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