A man was hospitalised after trying to stop an assault outside the Guildhall in the early hours of last Friday.
The incident happened at Whittaker Street shortly after 2am on Friday, December 4.
A man aged in his 29 intervened to try to stop an assault on another man.
He was punched once in the face and fell to the ground, hitting his head as he did so. He was taken to hospital.
The alleged offender was wearing a dark top and green trousers, and walked off along Shipquay Street towards the Diamond after the incident.
Information can be given to police at Strand Road by telephoning 101, or to Crimestoppers anonymously on 0800 555 111.