A confrontation between two people in a bar quickly turned into a mass brawl, police have said.
The violent mayhem in the pub in Leeds was captured on CCTV in footage officers have released to try to round up those involved.
West Yorkshire Police said they were called to the Arc bar in Ash Road, Headingley, shortly after 12.30am on Saturday morning.
A spokesman said around 20 men were fighting with each other and using glasses and chairs as weapons.
Detective Inspector James Entwistle said: "The situation appears to have started with a confrontation between two people that has then quickly turned into a mass brawl.
"These were some very ugly scenes where people were punched, kicked and stamped on and glasses and chairs were thrown.
"The video shows how there was very little that the bar's security could do to control the situation and it was clearly a shocking thing for staff and other people in the bar to witness."
Mr Entwistle said: "We are treating this as an incident of violent disorder and are carrying out inquiries to identify those involved.
"We do have very clear CCTV footage that captures the whole incident and are working through that in detail to identify the main suspects.
"We would like to hear from anyone who knows the identity of any of those shown fighting in the clip we have released or who recognises themselves as a witness."
The spokesman said a 28-year-old man was found with a wound to the head and was treated at hospital.
A 21-year-old man was arrested later on suspicion of violent disorder and has been released under investigation.