Riot police in the Polish city of Wroclaw used a water cannon to stop trouble between Chelsea and Real Betis fans before the Uefa Conference League final.
Hundreds of fans clashed in Wroclaw Market Square on Wednesday afternoon - with the Lower Silesian Police saying 28 supporters had been arrested.
Police indicated that number would rise, with arrests of fans involved in the incident "ongoing".
The final of the the third-tier European competition is at 20:00 BST at Stadion Wroclaw.
Local authorities estimate that more than 70,000 fans have arrived in the city in the south-west of Poland for the final, a number that covers ticket holders and those attending fan zones and other events around the city.