Police were called to Barthorpe Post Office on Stainbeck Road in the Chapel Allerton area of north Leeds just before 9.30am yesterday (Sat July 30).
Insp Graham Hawcroft, of West Yorkshire Police, said: “A male wearing a mask has gone into the shop with a firearm of some description.
“He made off on foot with a quantity of money from the till. A male has been arrested overnight and is being questioned by police.”
Any witnesses or anyone with information is asked to call police via 101.
Or call Crimestoppers, anonymously on 0800 555111.