At 5.35pm, the robbers were seen running from Hester Clarke - the county jewellers - on Cambridge Street.
They had left a car outside the shop with the keys in ready for a quick getaway.
A quick-thinking passer-by took the keys from the car and ran into another shop near to the scene to take refuge and call police.
The robbers jumped into the car, but upon seeing the keys had gone missing, they had to make their escape in another vehicle parked further away.
It is not known what was stolen at the moment, but Heston Clarke describe themselves on their website as experts in diamonds, fine jewellery, gold, watches and clocks, and modern and antique silver.
Read the full report with eye-witness accounts in tomorrow's Bucks Herald.Did you see the robbery yesterday? Contact our reporter on 01296 619767 or click here to email.