Fans of Chelsea and Real Betis have clashed ahead of Wednesday's Conference League final in Polish host city Wroclaw, authorities said, warning they would clamp down hard on violence.
Footage on social media showed fans in the city centre hurling chairs at each other and taking part in running battles on Tuesday evening.
Polish police said they were attempting to identify fans involved in one incident and had detained a Spanish citizen after a scuffle on Tuesday night. A further three Spaniards were detained in the early hours of Wednesday after they took down UEFA Conference League Final flags.
Last week’s Europa League final between Manchester United and Tottenham Hotspur was also marred by clashes between supporters ahead of the match in Bilbao.
"Zero tolerance for violence on our streets!," Polish Prime Minister Donald Tusk wrote on X.
"I thank the police for their decisive actions against the hooligans in Chelsea and Betis shirts in Wroclaw. We warn you: if necessary, the police will be even more ruthless today!"
'We're not scared'
Polish police said everyone could feel safe thanks to a large police presence saturating certain areas.
Chelsea, twice Champions League winners and six times English top-flight champions, arrive at the final having secured fourth place in the Premier League on Sunday, giving them Champions League football next season.
Real Betis supporters, meanwhile, are revelling in reaching a European final for the first time in the club's history and the chance to overshadow local rivals Sevilla with their own chapter of continental success.
Hours before the game on Wednesday afternoon, under steady rain, most fans were brushing off the violence.
"There is always someone bad in a crowd of 50,000 people," said Betis supporter Juan in Wroclaw’s old town square.
"We are loving every second of this , we’re going to celebrate afterwards, and we hope Chelsea fans will cry."
Another Betis fan, Maria, added: "We’re not scared, we’re very happy. Most of us just want to enjoy the football and the match."