Two men carried out a citizens' arrest on an armed thief who had stolen the day's takings from a shop in Bondi Junction, police say.

The pair managed to knock a stun gun out of the man's hands and overpowered him before police arrived a short time later and arrested the 21-year-old on Waverley Street on Monday afternoon.

Police said the two men had witnessed the robbery occur on Oxford Street at 3.15pm on Monday, when a 21-year-old woman was confronted as she left a shop carrying the day's takings in a plastic bag.

Police allege the thief snatched the bag out of her hand and began running east on Oxford Street.

The two men gave chase and caught up with him Waverley Street, where it is alleged he pulled out an electronic control device and activated it.

"The two witnesses managed to overpower the man, knocking the device out of his hand," police said.

Police officers arrived moments later and arrested the man.

He was taken to Waverley Police Station where he was charged with stealing from a person, and possessing or using a prohibited weapon.

He was refused bail, and is due to appear in Waverley Local Court on Tuesday.

No one was injured during the incident, and all money was recovered.