The fans were attacked by men armed with baseball bats and batons
The Irish Football Association's president has said he believes Northern Ireland fans were victims of an unprovoked attack in Slovakia.
It is understood two fans were knocked unconscious by a group of 12 men, thought to be from a security firm.
The supporters were beaten with batons, knuckle-dusters and baseball bats in the Dubliner bar in Bratislava.
They were part of a contingent in the city for the Northern Ireland-Slovakia match to be played on Saturday.
IFA president Raymond Kennedy said: "I am absolutely appalled that our fans, with the record (of good behaviour) that they have, cannot come away here and get into a pub and enjoy themselves.
"As far as I'm concerned, it has been an unprovoked attack and my thoughts are with the two guys who got beaten up - there's one that's lost five teeth.
"It's not a very nice thing to happen to come away here to support Northern Ireland and go out for a drink and you end up going back minus five teeth."
One fan, Matthew Ogle, said they did nothing to provoke the assault.
"We were all singing, having good fun and the doormen had a problem and moved in and started on my friend, just came in and took everybody out of it," he said.
"There were weapons mentioned, there were baseball bats, knuckle-dusters and stuff like that. It was scary."
Portadown Northern Ireland Supporters Club secretary Kyle Quinn said he witnessed the incident.
"I could see a couple of guys just lying prostrate on the ground and one guy in particular was lying over this guy who was obviously out cold," he said.
"These guys were just laying into them. There was one baseball bat and there was another fellow I saw with a white tee-shirt and a pink collar, he seemed to get attacked with, I think, a knuckle-duster."
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