A robber in a black face mask raided a city centre building society, stealing a "substantial" amount of money.
The man entered the Nationwide on Albion Street, in Leeds, at 10.30 BST on Monday, and produced a note demanding money.
He was brandishing a bag, and a cashier believed she saw what may have been the outline of a handgun inside.
No-one was injured during the raid, and the suspect fled through the Trinity Leeds shopping centre.
Police are now investigating and want to hear from anyone who took photos or video.
Det James Entwistle said: "Incidents like this are thankfully rare.
"It happened on a busy street in the centre of Leeds and people will have seen the suspect leaving the premises and his movements afterwards."
The suspect, who is thought to be in his 40s, has been described as short and stocky.
He was wearing a dark hooded top and had a 'fade effect' shaved into the sides of his hair.
Anyone with information is asked to contact West Yorkshire Police or Crimestoppers.