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A knifeman pulled out a large blade and demanded money from staff at a convenience store in front of two young children, police said.
Up to 10 people, including the two children aged under five, were in the One Stop shop in Copnor Road, Portsmouth, Hampshire, when the robber walked in with a scarf round his face.
One person challenged the armed man but he continued to make threats until a shop assistant handed over the contents of the till, about £350 in cash.
He then fled at around 7.30pm yesterday. Hampshire Police appealed for witnesses to come forward, particularly the member of the public who tried to remonstrate with the robber.
Detective Constable Rachael Hannam, of Fratton CID, said:
"This was a very distressing incident which occurred in front of a shop full of people, including two very young children."
The offender is described as white, aged in his 20s, and wearing a navy blue jacket.
Anyone with information is asked to call Hampshire Police on 101 or the Crimestoppers charity anonymously on 0800 555 111.