The 35-year-old was assaulted after intervening in a dispute between two groups of people near Bentley’s bar on Godwin Street in the centre of Bradford in the early hours of today.
He was taken to hospital, where his condition was tonight described as critical as he received treatment for a serious head injury.
Police are appealing for witnesses to the attack, which took place some time between 1.30am and 2.15am.
Bradford District CID’s Det Insp Matt Walker said: “Given the time that this assault has happened, there will have been a number of people in the area, either making their way between bars or heading home.
“It may be that you witnessed some people fighting but at the time thought nothing more of it and didn’t believe it was worth reporting.
“We have a man fighting for his life in hospital and need anyone who has any information, no matter how insignificant it seems, to please come forward.”
Anyone with information is asked to call Bradford District CID via West Yorkshire Police’s 101 number or Crimestoppers anonymously on 0800 555111.