Sydney robber chased and held by onlooker

An armed robber's getaway in Sydney's west has been foiled by an onlooker who chased and held him until police arrived.

The masked robber threatened staff with a hammer at a pawnbrokers in St Marys before smashing glass cabinets and running off with jewellery, NSW police say.

But a witness chased the burglar into a nearby car park and restrained him until police arrived.

A 21-year-old man was charged with robbery while armed and is due to face Parramatta Bail Court on Friday.