Two men threatened a family with knives as they shopped in North Street, Brighton.
The suspects got talking to a woman of 38 and her two teenage children outside Boots, near the junction with Queen’s Road.
When the woman’s husband joined them with the couple’s other son and asked his wife if she was ok the conversation became less friendly.
The suspects then each produced a knife from their clothing, pointed it towards the family and walked off towards the Clock Tower.
Sussex Police said that the incident happened shortly after 6.30pm on Wednesday (29 December).
PC Chris Welsh said: “This was a nasty incident involving a family having a day out at the sales.
“Although no one was hurt we are very keen to trace the two suspects.”
One of the suspects is described as white, about 18 years old, 5ft 8in tall, of lanky build with blond hair and blue eyes.
He was wearing a white top and black trousers and appeared to be carrying what looked like a kitchen knife in his trouser pocket.
The other suspect, who was carrying a knife in a front zip pocket of his black jacket, is described as South East Asian, 19, about 5ft 8in tall and of broad build.
He had a distinctive goatee style beard and was wearing a black hat and black jacket.
If anyone saw the suspects or the incident they are asked to contact Sussex Police on 0845 6070999 or the independent charity Crimestoppers anonymously on 0800 555111.