A female cashier was also pushed as two thugs raided the Great Southsea Street shop just before 10pm on Saturday, stealing cash from the till.
Police believe the robbery was carried out by two men, who fled the scene.
No one was injured but officers have released CCTV images of two men they want to speak to in connection with the incident.
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Detective Sergeant Paul Stenton said: ‘We know that the images are not great, but we’d like to speak to these two men and hope you might recognise them from their clothing.
‘If you think you know who they are, please let us know, as we believe they will have information which could help our investigation.
‘We’d also like to hear from anyone who has any private CCTV in the area or who saw anything suspicious that evening.’
CCTV shows one of the men wearing a dark Adidas tracksuit, a grey gilet and a dark scarf and baseball cap.
The other is seen wearing a khaki green or grey coat, with a dark hood covering his head.
He appears to be wearing tracksuit bottoms and trainers with white soles.
Anyone with information on the raid should call police on 101, quoting the crime reference number 44190412662.
Alternatively, call Crimestoppers anonymously on 0800 555 111.