One member of the gang is said to have grabbed a pint glass off the bar and smashed it over the victim's head before running away from the scene.
The victim, who was "covered in blood", was taken to the ERI before being moved to St John's Hospital for plastic surgery on the deep wounds.
Publican Silvana Murray said she was "devastated" that something so horrific could happen in her normally peaceful pub.
She told the Evening News: "In all my years, since buying the pub in 2002, I have never seen anything like this. I am devastated and feel physically sick thinking about it.
"We are a peaceful pub where different football supporters and all different races can mix without any bother.
"At around 9pm, I believe some words were exchanged between the two gangs and racial slurs were thrown about, then the guy attacked him with a glass. I'm not entirely sure how it came about as staff were changing over at the time.
"It was horrific and the guy was covered in blood. His girlfriend said he's been taken to St John's Hospital where he's getting some plastic surgery.
"He's a regular here and a nice lad."
She added: "We have given police CCTV footage of the incident. We want this person caught."
A police spokesman said: "The suspect is described as male, white, 5ft 10ins tall with shaved fair hair. He was wearing a black jacket, a red t-shirt, dark jeans and coloured trainers with the numbers 006 on them. He is believed to be between 25 and 30-years-old.
"Anyone who was within the pub and witnessed anything suspicious or who has information that can assist in identifying the male responsible is asked to contact police immediately."
It is not the first time in recent months that violent, anti-social behaviour has been reported on Leith Walk.
The owners of the nearby Word of Mouth Bistro said they had witnessed a stabbing, glass bottles being thrown at passers-by and several incidents of theft from their own premises.
Owner Kirstie Campbell, who has been forced to call the police more than 10 times in 2010, said a man stumbled into their premises last March after being stabbed in the stomach.
She said: "We were just closing up when a guy came up and knocked on the door.
"He pulled his t-shirt up to reveal a large, deep wound on his stomach so we got him in and called an ambulance. He had seen a couple having an argument and the guy has stabbed him then ran off.
"Sadly it's not an isolated incident."
Leith councillor Gordon Munro said: "Evidence from police indicates there are less violent incidents within the area nowadays, but if people aren't reporting anti-social behaviour I would encourage them to do so, so that they give us a true picture of what is going on."
The Scotsman
