A man sustained serious injuries following clashes after the Bristol City's game with Millwall on Saturday.
Great Western Ambulance Service said the man, who is reported to have head injuries, was taken to the Bristol Royal Infirmary.
Two other men were treated for minor injuries, a spokeswoman added.
Police are appealing for information about an incident in which a coach with Millwall fans on board was attacked by rivals in Bristol's Cumberland Basin.
It is not clear at present if the injured man sustained his injuries during that clash.
Avon and Somerset Police say six people were arrested for minor public order offences connected to the game.