Detectives from North Yorkshire Police launched an inquiry and believe the 35-year-old man could have suffered serious head injuries as long as two hours before he was discovered.
He was found lying unconscious in the playing fields behind Burton Green Primary School in the Clifton area of the city at about 2am on Tuesday. He was taken to York Hospital where he remained in a coma yesterday.
Three local men aged 19, 21 and 36 have been arrested in connection with the incident and were last night in police custody for questioning.
Inspector Ian Dyer, of York CID, urged anyone with information to come forward, and added: “As a result of this incident there is a man fighting for his life in hospital.”