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A couple were attacked while they sat in their car at the Aldi supermarket in Gillingham.
The pair, who have not been named, were attacked by a man who then demanded they hand over some cash.
It happened between 7.30pm and 8pm and their attacker left empty handed and ran off in the direction of Connaught Road.
Officers investigating Monday's attempted robbery are appealing for witnesses.
The suspect is black, 5ft 9in, of stocky build, and wearing a grey hooded top with the hood up.
Officers are keen to speak to anyone with information about the incident or can identify the suspect.
They would also like to speak to anyone in the area who may have dash-cam footage of the incident.
Witnesses are asked to call the appeal line on 01634 792 209 quoting reference number XY/65388/17 or Crimestoppers on 0800 555111.