A boy who is believed to be just 15 years of age dragged a pensioner from the road and tried to snatch the bag he was carrying.
Northamptonshire Police say the incident happened at 8.45am, on Friday, when a 69-year-old man was walking towards town, along London Road.
Two boys then approached him as he passed by a bus stop, in Cotton End opposite the Old White Hart pub, and then grabbed his bag.
He was pulled him behind a nearby billboard, when a member of the public intervened and shouted at the robbers, who ran away.
One of the offenders is described as a 15-year-old boy, who was 5ft 7in tall, of medium build and was wearing a grey hooded top.
No description is available for the second robber.