Robbers armed with axes and bats rode motorcycles through and indoor shopping centre and raided a jewellery store.
Six robbers on three bikes including a white Triumph Daytona 675 rode through Brent Cross Shopping Centre at 10.15 this morning.
The pillion passengers got off and smashed the windows of Fraser Hart, stealing watches and jewellery. The suspects wore helmets with dark visors.
Police later found the bikes abandoned.
Det Supt Mark Strugnell said: "We are appealing to anyone who may have seen the sequence of events at the shopping centre, which had only just opened its doors to its first shoppers only moments before, and to anyone who may have seen the three motorbikes being driven from Brent Cross to Mill Hill."