The man was rushed to the Royal Adelaide Hospital where he remains in a serious condition.

Police were called to the incident just before 5am.

Crime Scene Investigators closed Hindley Street between Rosina Street and Blyth Street and have asked people to avoid the area.

The suspect is described as a Caucasian man with a shoulder-length blonde mullet.

He was last seen walking east on Hindley Street before turning north onto King William Street.

Police are asking for anyone with any information to call Crime Stoppers on 1800 333 000 and say they may remain anonymous.