Police say the person's injured but in a stable condition, while three arrests have been made as investigations continue.
Officers were called to a pub in St Peter Port, that they're not naming, at 4 o'clock yesterday afternoon (16 December).
Police describe the mass fight in the bar as 'serious.'
They're asking people not to share phone footage of the fight on social media, as it's 'particularly violent' and 'many people may not want to see it.'
Police say they have copies and are analysing them as part of their investigation.
They say the licensee of the pub has given them their full co-operation.
Police want anyone who was in the bar at the time of the fight, and has not already spoken to officers, to make contact with them.
Three people have been arrested and the investigation continues.
Police were called to a number of incidents in St Peter Port last night and a further person has been arrested over an alleged assault at the North Plantation.
Officers were posting on social media as part of their #TweetontheBeat operation, which gives people an insight into their work in the busy lead up to Christmas.
Possible job cuts at RBSI in Guernsey and Jersey
Guernsey students beat England's A level pass rate
Guernsey community unites out west for solar eclipse
19 months of traffic lights on busy Guernsey road
New Dean of Guernsey has Alderney connections
Guernsey's youngsters concerned by the high cost of living
A solar eclipse and meteor shower over Guernsey tonight
Average Guernsey home selling for more than £600,000
