Fans clashed in Upper Bangor ahead of a Welsh Cup derby.
Police say one officer was injured after supporters managed to break through a cordon at just before 7pm on Saturday night (26th January) ahead of the fourth round game between Bangor City and Caernarfon Town.
Flares were let off during the stand off, and at one point, the public was told to stay away from Holyhead Road while fans dispersed.
Superintendent Richie Green said the disorder was brought under control quickly but one of his colleagues was taken to hospital.
He added: ”It is a shame that a small number were intent on causing problems, despite police presence. As always, the vast majority of those attending are law abiding football fans and I’m appreciative of their patience whilst the policing operation was in place.”
Detectives are appealing for anyone who witnessed the disturbance to contact Bangor police station on 101, quoting reference no. X011746.
Around 2,500 people were at Nantporth for the first competitive meeting between the Gwynedd clubs since 2015. Caernarfon won 2-1 with goals from Jamie Breese and Gareth Edwards.
Jordan Piggott equalised just before half time for City – but the Canaries held out to reach the quarter finals. The draw will be made on S4C’s Heno on Monday night.