Police are appealing for information about the fight that broke out near the Havre des Pas Lido on Monday evening (18 May).
It involved up to 20 people, but the force thinks only a small number of those were physically fighting.
Two boys were arrested. A 16-year-old on suspicion of grave and criminal assault and a 17-year-old on suspicion of a breach of the peace.
They've been released on bail while enquiries continue.
Anyone who saw what happened or has any information is asked to call police on 612612, Crimestoppers anonymously on 0800 555 111 or via the online form.