A Northern Ireland fan said he was left "dripping with blood" after being caught up in violent skirmishes in Nice on Saturday night.
Trouble broke out when supporters came under attack from French hooligans.
Six Northern Ireland fans were injured, including Adam Carr.
He described how yobs in hooded tops "stormed the crowd" and then threw missiles.
Mr Carr was taken to hospital and needed four stitches.
The trouble started at around 11pm local time on Saturday night. A senior PSNI officer in France to support local police at Euro 2016 stressed that Northern Ireland and Poland fans were not to blame.
Supt Nigel Goddard said: "It was clearly the French supporters that came amongst the Polish and Northern Ireland fans and started this.
"There certainly wasn't any trouble or animosity between the Northern Irish and Polish fans."
Between 20 and 30 French hooligans arrived outside the pub where Northern Irish and Polish supporters had been drinking and singing.
Bottles were thrown and punches were exchanged.
Around a dozen of the Northern Ireland contingent retaliated and one was arrested.
Mr Carr described how he had tried to escape the bottles and debris raining down by heading back inside the bar.
"After a couple of minutes I realised [one had] sliced me, I was dripping with blood - I had to go to hospital and got four stitches," he added.
"There were about six or seven Northern Irish boys in the ambulances, boys with their heads cut open." Six Northern Ireland fans and one Polish fan were injured.
A photograph showed one of the Northern Ireland supporters holding the hand of the injured Polish supporter in a gesture of friendship as they waited in hospital. Supt Goddard said a small number of fans had responded to the provocation, but stressed it was a "short and minor incident", and that relations between both sets of fans had been "excellent".
Yesterday there was no hint of trouble as supporters mingled in the sunshine ahead of the match.
Rival fans sang and posed for photographs together, with many making the final walk to the stadium side by side.
England have been warned by UEFA that they risk elimination from the Euro 2016 championships, along with Russia, if their supporters are involved in a repeat of the violence which left 19 Britons hospitalised in Marseille on Saturday.