The incident, which occurred at approximately 3.30am, led to deployment of some 100 riot police with shields to a corner of the square to cordon off a section of fighting fans.
It is believed the altercation began between Polish fans and a number of Croatian fans from Zagreb. Bottles and large parasols for the outdoor bars were thrown. Nobody was seriously injured in the confrontation, which lasted for about 15 minutes before the riot police dispersed those involved.
A total of 17 people were arrested, including three Irish fans. They were detained for damage to property and for infringing on the dignity of a Polish police officer.
Commenting on the incident, Ryszard Grobelny, the mayor of Poznan, said the quick actions of the police prevented the incident from escalating into a more serious one.
“Luckily there were no serious injuries. There was some minor property damage. To be frank, when you have a large group of fans and many litres of beer, incidents like this can happen,” he said.
However, Mr Grobelny stressed that, with almost 50,000 people in the city centre for the opening two days, there had been virtually no public disorder.
“If you consider the scale of the event, this incident was pretty small. Of course we cannot guarantee that more things like that will not happen, but the police will react swiftly and harshly to every incident. I hope there will be nothing like that again,” he said.
Mr Grobelny said the incident was just a small blip in an otherwise party atmosphere.
“We have 20,000 Irish, 8,000 Croats, and everyone is having fun and mixing well. We are very happy to see Poznan green at the moment and it’s amazing to see just how well everybody is enjoying themselves,” he said.