The six robbers were armed with axes and bats and drove through the Brent Cross Shopping Centre at 10.15am, reportedly targeting Cartier and Rolex watches from the Fraser Hart store.
Having smashed the windows, they carried their haul away on their bikes towards Hendon Central. The bikes were later found abandoned near a golf course.
Detective superintendent 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 moments before, and to anyone who may have seen the three motorbikes being driven from Brent Cross to Mill Hill.”
A man was treated for shock but no-one else was injured.