A 34-year-old man was taken to the John Radcliffe Hospital after being found unconscious with a head injury in Bulwarks Lane, Oxford.
A Police Community Support Officer (PCSO) was on patrol in the city centre when she discovered the man between 10am and 11am on Saturday. He is currently in a stable condition.
Det Insp Colin Payne said: "We are investigating all possibilities and considering all the options around what had happened to this man. If anyone saw this man collapse then I would urge them to contact the police.
Anyone with any information can contact Det Con Steve Clarke at Oxford police station via 0845 8 505 505, or if you don't want to talk to the police or give your name, call the Crimestoppers charity on 0800 555 111.
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