Two teenagers snatched a pensioner's handbag but fled empty-handed after a passer-by gave chase.
The youths, aged between 14 and 16, approached the woman in Tarring Road, Worthing, and asked her for a light.
One of them grabbed her bag and they ran off.
The incident, at 5.30pm on February 19, was seen by a man in a car who chased them. The pensioner was unhurt.
Sergeant John Skelton said: "This was a callous and heartless crime against a vulnerable member of society."
The youths were both 5ft 10in tall, of medium build and were sharing a large bottle of orange fizzy drink.
One was wearing a light quilted puffer jacket and woollen hat.
Anyone with information should contact Mr Skelton on 0845 6070999.
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