A 42-year-old man was knocked unconscious.
The assault happened outside Pizza Town in Biggleswade High Street at around 2.50am on Sunday, December 15.
A white man in his mid-thirties struck up a conversation with the victim and then, in an apparently unprovoked attack, hit him in the face.
The victim fell to the floor unconscious, cutting the back of his head and causing a large lump on his face. The emergency services were called immediately and he was taken to Lister Hospital, where he was given stitches.
The offender was about 6ft 2ins tall. The police have arrested a man but they need witnesses to come forward.
Detective constable Mike Armstrong said: “We know that several people saw some or all of what happened but as yet have not come forward to the police.”
Call Det Con Armstrong on 01234 275227 or phone the non-emergency number, 101.