Northern Territory Police are today calling for anyone who may have witnessed an assault on Smith Street in the Darwin CBD at 4:50am this morning.

Detective Senior Sergeant Chris Board from the Serious Crime Squad said “Detectives are currently investigating the circumstances surrounding an assault upon a 23-year-old male who was punched to the head causing him to fall, hitting his head on the ground and rendering him unconscious.”

He was taken to the Royal Darwin Hospital where he is in a serious but stable condition.

A 20-year-old male was taken into custody a short time later.

“Detectives have reviewed CCTV and are aware a number of witnesses were present leading up to, during and after the assault. We would like these persons to make contact with police,” said Detective Senior Sergeant Board.

"It appears to be cowardly assault resulting in the potential for fatal consequences,” said Detective Senior Sergeant Board.

Anyone who may have witnessed this assault is urged to contact police on 131 444 or Crime Stoppers on 1800 333 000.