Police were called at about 5.45pm on Saturday to reports that a man had been seen in Preston town centre with a knife.
At the same time officers were alerted by the ambulance service that they were attending to a man on Cheapside who had suffered a stab wound.
The man, a local 17 year old, was taken to hospital with a stab wound to the leg. The injury is not life threatening.
No other injuries have been reported as a result of the incident and enquiries are on-going to arrest the offender.
Detective Inspector Steve Monk, of South CID, said: “I appreciate this will have caused distress and concern to those who witnessed it and I would reassure people that we are treating this as an isolated incident and an investigation is underway to identify and arrest the offender.”
Anyone with information should call police on 101 quoting log number 1070 of Saturday, September 8th or independent charity Crimestoppers on 0800 555 111.