Two men who “befriended” a teenager left him unconscious with a suspected broken jaw.
The 18-year-old victim, who was alone at the time, is believed to have been stamped on by his attackers.
They struck as he walked down Cheapside in Preston, close to the Flag Market, after a night out.
They spoke to him and were friendly but suddenly attacked him for no reason.
It happened between midnight and 1am on Saturday into Sunday.
The victim, a car valeter from Walton-le-Dale, suffered severe facial cuts and a broken cheekbone.
Detectives, who were alerted by ambulance crews, today appealed for anyone with information to come forward.
Det Sgt Martin Pearson, of Preston CID, said: “ There is genuinely no motive – this was an unprovoked attack.
“The injuries he suffered are consistent with a sustained attack involving stamping, which is particularly nasty.”
Anyone with information can call 101 or Crimestoppers on 0800 555111.