A crime in progress was discovered by officers in a police car on routine patrol who pulled onto a garage forecourt in Sussex.
Two men in balaclavas were stealing property from the Tesco Express shop next to the filling station in Redkiln Way, Horsham, shortly before 12.30am on Friday 6 July.
The officers called for assistance from other units, and one man ran off and is still being sought, but his accomplice, a 26-year-old man of no fixed address, was arrested on suspicion of aggravated burglary and is currently in police custody.
Anyone who saw what happened or who may have any other information concerning the incident is asked to contact Sussex Police online or by phoning 101, quoting Operation Latch.
Alternatively please contact Crimestoppers or phone the independent charity anonymously on 0800 555111.