A woman in a nightdress has been found seriously injured in an alleyway in Rochdale town centre.
Officers were alerted by paramedics who found the woman in a side street between Yorkshire Street and the town's police station in the early hours.
The 20-year-old had suffered serious head and facial injuries but Greater Manchester Police said it was unclear whether she had been attacked.
She was taken to hospital for treatment and remains in a critical condition.
Det Insp Vicky Shaw said: "Unusually this woman was in a nightdress when she was found and I am certain someone will have seen her at some point.
"It is too early to say she whether or not she has been attacked as we simply do not know how she came to be injured.
"I am keen to hear from anyone who saw this woman at any point last night or might know what happened to her to come forward."