The sex attack took place as the woman walked home alone
The victim, who was in her 50s, had been walking near a grassed area in Langley, Berkshire, when the horrifying assault took place.
Police say a young man approached her from behind before carrying out the sexual attack.
The shocked woman challenged the youth, and thankfully was able to flee the scene.
Police combed the Langley area of Slough today
Detectives were today spotted combing the area in Trelawney Avenue for clues to discover the identity of the sex attacker, who they described as aged in his late 20s or early 30s with blonde hair that was longer on the side.
A spokesman for Thames Valley Police declined to give more details of the attack, but she said the assault was serious although not a rape.
And she appealed for anyone, especially taxi drivers in the area, to contact police if they remembered seeing anything suspicious.
The woman was grabbed from behind as she walked through a park
We are appealing for witnesses to come forward after a woman was sexually assaulted in Langley.
Thames Valley Police spokesman
She added: "We are appealing for witnesses to come forward after a woman was sexually assaulted in Langley.
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“At around 2.20am today, the victim, a woman aged in her 50s, was walking near a grassed area off Trelawney Avenue when a man approached her from behind and sexually assaulted her.
"She challenged the man and she ran away from him. The offender is described as white, 5ft 10ins to 5ft 11ins tall, aged in his late 20s to early 30s and of medium build. He had short blond hair which was longer on the sides."
Police appealed for witnesses following the sex attack
Detective Inspector Jason Kew, from Maidenhead CID, added: "I would like to speak to anyone who saw a man acting suspiciously in the area at around the time of the incident or anyone who has any information which could assist our inquiries.
"A taxi company is based in area and it would have been busy at the time picking people up from pubs and nightclubs.
“I would like to hear from any drivers or passengers who noticed anything out of the ordinary which could assist the investigation.
"Specially trained officers are currently supporting the victim. Detectives and uniformed officers are working in the area to identify and locate the offender.
“Uniformed patrols will take place overnight as officers will continue to gather information and reassure the community."