Merseyside Police received a report of a concern for safety of a 16-year-old female on Buckingham Street, near Great Homer Street, in the grounds of Lifestyles gym.
She was found unconscious and is believed to have collapsed between 9.30pm and 10.45pm yesterday (Sunday 21 November).
The female was taken to hospital, where she currently remains in a critical but stable condition.
Extensive CCTV, witness and forensic enquiries are being carried out to establish the full circumstances.
Detectives are urging anyone with any information to come forward.
Detective Sergeant Timothy Farley said, “We are in the extremely early stages of this investigation and are working to establish the full circumstances of how a young woman was found unresponsive.
“If you were in the Great Homer Street area late last night and saw anything or anyone suspicious then please let us know.
“Similarly if you were driving in the area and have dashcam footage then please review it and get in touch if you see anything. Information you hold could prove vital to our enquiries.”
Anyone with any information is asked to DM @MerPolCC or message Merseyside Police Contact Centre on Facebook, quoting reference 21000810802.
You can also pass information anonymously via Crimestoppers on 0800 555 111, or contact them via their online form at: https://crimestoppers-uk.org/give-information/give-information