A man arrested on suspicion of assault occasioning grievous bodily harm (GBH) has been released under investigation.
The 25-year-old from Peterborough had been arrested in the early hours of Sunday morning after a woman was assaulted.
Officers were called to reports of violence in the Northminster area of Peterborough at around 1.58am.
They attended the scene, together with paramedics, where they discovered a woman in her 40s had suffered a serious head injury.
The woman was taken to Peterborough City Hospital in a serious but stable condition.
Officers are appealing for witnesses to the incident and are urging anyone with any information to contact police.
Anyone with information about the incident should call 101, quoting crime reference number 35/51691/19, or report online at www.cambs.police.uk/report.
Alternatively, they can report it anonymously by calling Crimestoppers on 0800 555111 or visiting https://www.crimestoppers-uk.org.