A man was taken to hospital after being punched and kicked by a group of men on Above Bar Street.
The victim, a man in his 20s, required treatment after sustaining a fracture to his face and injuries to his hands, back and eye.
Hampshire Police are now looking to speak to three people who the force says were seen in the area at the time of the incident.
Officers said that the victim was walking along Above Bar Street at around 5.30pm on Wednesday when he was approached by three unknown men.
They said that a brief altercation took place before the men parted ways.
Just over an hour later, at around 6.40pm, the victim was approached by the same trio outside of O'Neills, on Above Bar Street, when the alleged attack occurred.
A spokesperson said: "Officers investigating this incident have been carrying out enquiries in the area and would now like to hear from anyone who recognises these three men, who were seen in the area at the time, and may have information to assist our enquiries.
"We would also like to hear from anyone who was in the area at the time of the assault and saw what happened.
"Anyone with information is asked to call 101 quoting reference 44240204154."
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