A robber attacked a victim who was working on a St Albans market stall and stole two gold rings from her, before being chased by a group of pupils.
Police were called at 1.24pm yesterday after the robber pushed the victim to the floor in St Peter's Street.
Anne Main, Conservative parliamentary candidate for St Albans, was outside the cathedral when she heard a girl in school uniform shouting and chasing after the suspect.
Mrs Main said: "A group of St Albans School pupils had seen this man attack a woman outside Boots and steal the rings off her fingers.
"We got a photograph of him and we chased after him as well.
"I can't commend the pupils enough, particularly this young woman, for their quick-thinking. She was determined to try to bring him to justice.
"They gave chase without thought of their own personal safety."
The suspect has been described as an Asian male aged between 20 and 30 years old, with short dark hair.
He was said to be wearing a royal blue Puffa jacket with a fur hood, dark jeans and white-soled trainers, and was carrying a red and white striped hat.
Enquiries are currently ongoing, and anyone with information is asked to report it as soon as possible.
You can report information online at herts.police.uk/report, or call Herts police on the non-emergency number 101.
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