It took place at Cobo Bay at around 18:30, during the first Balcony Gig of 2021.
Police have already arrested one person in relation to the incident. With so many people in attendance, it is hoped that witnesses will be able to identify any others who were involved in the altercation.
Anyone with information is asked to contact Guernsey Police on 222 222 or Crimestoppers anonymously on 0800 555 111.