A member of staff at Morrisons in Verwood had to be treated for injuries after getting involved with a customer issue.
The victim stopped a group of people who were having an altercation with another customer, before one of the group proceeded to assault the victim.
It is reported that the group then made off with stolen items and caused damage to the windows of the premises as they left.
Police Community Support Investigator Kate Hann, of Dorset Police, said: “The behaviour demonstrated during this incident is completely unacceptable and we are doing all we can to support the victim and identify those involved.
2PM UPDATE: An 18-year-old man from Hampshire is assisting officers with their investigation and is due to attend for a voluntary police interview.
The incident happened at around 8.50pm on Wednesday 4 October 2023 and anyone with information is asked to contact Dorset Police quoting occurrence number 55230159784.