A 19-year-old from Preston was chased and attacked by three men carrying weapons.
The group are said to have attacked the victim with weapons, knocking him to the ground in Church Street before kicking and punching him.
The man suffered a serious head injury and was taken to Royal Preston Hospital for treatment in the assault on Saturday, September 1 at around 5pm.
One of the alleged attackers was detained at the scene by a member of the public and later arrested by police. Two others made off from the scene.
Detective constable Richard Shires, of Preston CID, said: “We believe a number of shoppers were in the area at the time of the alleged offence and can provide us with information.
“If you saw what happened and have yet to speak to police, please contact us.
“We are particularly keen to speak to anyone who saw two men making off from the scene. Any information will be treated in the strictest confidence.”
A 17-year-old boy from Preston was arrested on suspicion of wounding and later released under investigation.
Call 01772 209803 or email firstname.lastname@example.org quoting crime reference SA1818521 with information.
Alternatively independent charity Crimestoppers can be contacted anonymously on 0800 555 111 or online at crimestoppers-uk.org.