A police officer died on Thursday in the Spanish city of Bilbao after clashes with Spartak Moscow fans ahead of a Europa League soccer game against Athletic Bilbao.
The police officer died in hospital, where he had been taken following a heart attack when the regional Basque Country police force Ertzaintza was trying to stop street battles between supporters of the two teams, the police said.
At least five people were arrested.
Spartak won the second leg of the round of 32 tie 2-1 but were knocked out of Europe's second-tier club competition 4-3 on aggregate after losing the first leg 3-1 in Moscow.
Russia hosts the World Cup this year and local authorities are putting strict security procedures in place to control hooligans, including Russian fans who caused trouble at the 2016 European Championship in France and other soccer events.
Organising body UEFA said it "strongly condemns the violent clashes", adding in a statement: "We are in contact with local authorities to obtain further information on these incidents.
"UEFA sends its sincere condolences to the family and colleagues of the police officer who passed away tonight."
"Our condolences to colleagues, friends and relatives of the Ertzainza (force officer) who died in an act of service," Bilbao Police wrote on Twitter.
Athletic added a statement of their own, saying: "Our heartfelt condolences to the family and friends of the deceased officer today. Athletic Club express their strongest rejection of violence generated around football."
At his post-match press conference, Athletic head coach Jose Angel Ziganda said, in quotes carried on social media: "We didn't know anything. We found out at the end of the game. I don't know if would have been better or worse to suspend the match."
