The boy was stopped by police in Aberdeen Drive, Armley using stop and search powers.
Police shared these images of a gas powered gun and a lock knife on social media after the incident.
-> Police name cyclist who died after crash at Cardigan Fields in LeedsA police spokesman put a post on the Leeds Crime Twitter account comparing the imitation firearm to a photo of a real gun. The tweet said: "Effective and proportionate use of stop search powers. Arrested. One of the weapons is real, one is an gas pistol, could you tell the difference?"
West Yorkshire Police confirmed that officers arrested a 15-year-old boy at 9.40pm on Tuesday, January 29.
The teenager was arrested on Aberdeen Drive on suspicion of possession of an imitation firearm and a knife.
The boy remains in custody today, police said.