The assault happened at about 7.30pm last night, Sunday, March 10, when the woman was attacked by a man, causing injuries to her head.
A man passing by stopped to help, allowing the woman to escape.
Officers investigating the incident are urgently asking the passerby and anyone else who saw the incident to get in touch by calling 101.
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Information can be shared by calling Northamptonshire Police on 101, or contact Crimestoppers anonymously on 0800 555111.