The 39-year-old victim was in a group of adults who were approached by a large gang of youths near the Debenhams store on Briggate.
He was hit and kicked by one of the youths before the gang made off on foot.
The incident happened at about 7.55pm last Saturday, January 16.
Leeds CID’s Det Sgt Amanda Wimbles said today: “This was a particularly nasty assault which has left a man in a serious condition.
“It happened on a very busy road in the centre of Leeds so a lot of people will have seen what happened either immediately before or during the incident.
“I would appeal directly to anyone who hasn’t yet come forward with information to do so straight away.”