At 3.59am today (Saturday), police received a call from ambulance staff reporting an assault had occurred outside the Revolution Bar in Cross Church Street.
Officers attended and found a 23-year-old man in an unconscious state.
He was taken to hospital. His injuries are said to be life-threatening.
A scene is in place in Cross Church Street for forensic examination.
The incident is being investigated by the Homicide and Major Enquiry Team, who are appealing for anyone who witnessed what happened, or who has footage that may assist to contact them on 101, or by using the LiveChat facility on the West Yorkshire Police website.
The log reference is 261 of 15 July.
Information can also be reported anonymously to the independent charity Crimestoppers on 0800 555111 or online.
