A robber tried to grab a teenager's handbag in Abingdon.
The 17-year-old girl was in Ock Street, near to the junction with Conduit Road, between 6pm and 6.45pm on Saturday when she was held by a man who tried to steal her bag.
A woman passer-by shouted at the man and he ran away.
Police have arrested a 49-year-old man on suspicion of robbery but are appealing for the woman who saw the incident to come forward.
She is described as white, about 5ft 4in tall, slim and in her 60s and wearing a long floral skirt with a brown jacket.
Det Con Julia Tuhill said: "I am keen to trace the woman who saw this happen as she could help me with my enquiries."
Anyone with information should contact police on 08458 505505 or speak anonymously to Crimestoppers on 0800 555 111.
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