Dorset Police are appealing for witnesses after an argument between two men in a car park in Littlemoor.
A 68-year-old man from Portland was taken to hospital with a dislocated right arm after the incident at 8.25pm on Thursday in the Co-Op car park.
A 26-year-old man from Weymouth has been arrested and released on bail pending further enquiries.
Detective constable Steve Brown, of Weymouth CID said: "I understand that this argument took place in front of a number of members of the public and it may be that someone, perhaps several people, will be able to help with my investigation."
Anyone with information is asked to contact Dorset Police by calling 101.
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