A man was found unconscious at Mount Pleasant following a suspected assault.
Motorists reported that a man in his 20s was on the floor in Oxford Street by its junction with Grove Street at around 4.10am on Sunday.
Both police officers and the ambulance service attended and the man was taken to hospital with facial injuries.
He is currently under sedation and in a critical condition. It is initially believed that the injuries suffered are consistent with an assault.
Detectives believe that the man had been in Liverpool city centre prior to the unexplained incident.
Forensic, witness and CCTV enquiries are ongoing to establish the full circumstances of the incident and detectives are keen to speak to anyone with information.
Detective Inspector Lee Schofield said: “At this early stage it is unclear exactly what happened so I would urge anyone who was in the area of Oxford Street around 4am and saw anything suspicious to get in touch.
“We are keen to speak to every member of the public who kindly stopped to assist and report this incident so if you were one of those people and have yet to provide a statement, please call.
“From our enquiries, we believe that the injured man may have been out with people in the Victoria Street and Dale Street area of Liverpool city centre at around 3.30am, from where he left to make his way home.
“If anyone was passing this area and saw a man in distress or any suspicious incident, we want to hear from you.
“Likewise, any passing motorists in either location who have dashcam footage of this incident may prove vital to our enquiries.”
Anyone with information is asked to call detectives on (0151) 7774036 or 101.
You can also call the independent charity Crimestoppers anonymously and for free on 0800 555 111 or via their online form.