Several roads were shut in Halifax after a 21-year-old was attacked.
The victim needed hospital treatment with serious, but non life-threatening injuries in the early hours of Friday morning.
Police closed roads in the centre while investigating the scene.
Silver Street, Waterhouse Street and John Street at the junction of Crown Street were shut on Friday morning while detectives scoured the scene for evidence.
They were alerted to the attack after the victim took himself to Calderdale Royal Hospital.
Anyone with information on the incident is urged to call 101 quoting 192 of August 19.
In unrelated investigations, police would like to speak to the people pictured in the image gallery below. The people pictured in the images may be witnesses as well as suspects. If you recognise anyone, contact police on 101 quoting the reference number on the image caption.