Three men smashed several display cabinets in the raid in Canterbury on Wednesday, Det Con Neil Martin said.
No staff were hurt, but it is believed a member of the public tried to stop the men as they left and suffered a minor head injury, he added.
He said the men, who were black and in their early 20s, left in a light blue Peugeot car that was parked nearby.
They were between 5ft 7ins and 6ft tall, and wore dark clothing with hoods over their heads.
In an appeal for information, Det Con Martin said the robbery happened at about 1545 GMT in Rose Lane.
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