Four jewel robbers have been thwarted by a 70-year-old member of the public.
The men, wearing black motorcycle helmets, raided Thurlow Champness jewellers at Abbeygate Street, Bury St Edmunds, Suffolk, at 1000GMT.
They ran from the shop with a bag of jewellery, but it was grabbed by a passer-by.
The offenders drove off on their motorcycles empty-handed. Two bikes were later found abandoned nearby on Long Brackland.
No-one was hurt in the raid. Officers and the force helicopter are searching the area for the offenders and inquiries are being made at the scene.
Det Insp David Giles said: "The fact that two motorcycles have been recovered suggest that a further vehicle was used by the offenders to leave the area.
"I want to hear from anyone who may have seen the motorcycles enter Bury town centre, or anyone with any information on any other vehicles that may be involved."
Anyone with any information is asked to call Suffolk Police.