South Wales Police say a 27-year-old man is in custody in connection with a robbery at a petrol station shop in Cardiff.
The incident took place at the Co-op garage on Llantrisant Road in the city on Monday.
A man wearing a Halloween mask and carrying an axe threatened a female member of staff before demanding cash.
The arrest follows another attempted robbery at a post office in Fairwater on Thursday.
Detectives say a demand was made for cigarettes in the shop at about 06.00 BST.
The man, from Fairwater, is being questioned on suspicion of robbery.