Two robbers with a firearm fled from a shop in Birmingham on Monday after snatching a carrier bag containing orange juice and a pack of cigarettes.
A shop worker was allegedly pistol-whipped as he locked up the Nine Wines Store, but fought off the attackers with a shop sign.
Armed officers responded after being flagged down by the 29-year-old victim in Handsworth at around 23:50 GMT.
A 26-year-old suspect was arrested at gunpoint near the Nineveh Road shop.
He was arrested on suspicion of robbery.
A suspected firearm was also seized.
The victim was taken to hospital but was discharged a short time later after being treated for a minor head injury.
Police have cordoned off the scene for forensic examination.
DC Phil Langstone, of the force CID in Harborne, said: "The victim was very brave in his attempt to fight off the attackers and the fact that armed officers just happened to be passing at that particular moment was a case of being in the right place at the right time."
Police are hunting a second suspect and have appealed for information.