Police are appealing for information about the assault at Safari nightclub, on Assembly Street in the city centre, at around 3.30am on Sunday.
The 38-year-old male victim, who is from Leeds, suffered serious head injuries in the attack. He was taken to Leeds General Infirmary and remains in hospital, where his condition has been described as stable.
British Transport Police, which has responsibility for 23 bars and nightclubs in the area around Leeds railway station, is appealing for witnesses to the attack.
Detective Inspector Mick Jackson said: “The man was punched in the face so hard he fell through an emergency exit and onto the floor where he was knocked unconscious during the altercation. He suffered serious head injuries and remains in hospital in a serious but stable condition.
“Such violent behaviour will not be tolerated and we will do all we can to bring those responsible to justice. We have launched an investigation and are reviewing CCTV and speaking to a number of witnesses.
“We know that the club was busy on Saturday night and we are hoping that other people may have witnessed the attack and I would encourage them, and anyone else who saw what happened, to please contact us.”
Anyone with information can call BTP on 0800 40 50 40 or text on 61016, quoting reference quoting ref 202 of 11/7.
Alternatively, details can also be given to the independent charity Crimestoppers anonymously on 0800 555 111.