An incident at a Tesco Express shop next to a filling station in Redkiln Way, Horsham, shortly before 12.30am on Friday (6 July) is being investigated.
A police car on routine patrol happened to pull onto the garage forecourt and discovered a crime in progress.
Two men in balaclavas were stealing property from the shop and officers quickly called for assistance from other units.
One man ran off and is still being sought, but his accomplice 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.