Police said two men wearing balaclavas stormed into the Elswick Road shop at 5.21pm wielding a gun and demanded staff hand over cash.
The pair left empty-handed and escaped in a grey Audi A3 that was found abandoned less than a mile away in Brook Place in Lea.
No arrests have been made at this stage but Lancashire Police are appealing for anyone with information or footage that could assist the investigation to come forward.
A police spokesman said: "We are appealing for information and CCTV and dashcam footage following an armed robbery in Ashton-on-Ribble area of Preston today.
"Officers were called to a Post Office in Elswick Road at 5.21pm following reports two males wearing balaclavas had entered with a suspected firearm and made demands for cash.
"Officers are keen to stress that it is being treated as an isolated incident at this stage and reassurance patrols have been stepped up in the area."
Anyone with information is asked to email [email protected] or call 101, quoting log 1025 of December 2, 2020.
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