A police officer has died during clashes between fans before Athletic Bilbao’s home Europa League match against Spartak Moscow at San Mames.
The officer from the Ertzaintza – the Basque Country’s police force – reportedly suffered a heart attack.
More than 500 officers were deployed for the fixture and reports say at least five arrests were made.
“Our condolences to colleagues, friends and relatives of the Ertzaina deceased in act of service,” Bilbao police said.
After the game, the La Liga club tweeted: “Our heartfelt condolences to the family and companions of the deceased officer today.
“The Athletic Club shows its strongest rejection of the violence generated around football.”
Videos of the violent clashes showed supporters firing fireworks at each other on the streets.
European football’s governing body Uefa said it “strongly condemned the violent clashes”.
The clashes came on the same day a Russian, wanted for attacking an England fan during the Euro 2016 riots in Marseilles, was arrested in Munich while travelling to the match in Bilbao.
Spanish prime minister Mariano Rajoy offered his condolences and condemned the violence.
“My appreciation to Ertzaintza, for their efforts to protect those who know how to enjoy the sport,” he tweeted.
La Liga president Javier Tebas added: “Ultra violence has claimed the life of a public servant – that’s enough. La Liga will continue with our policy of combating the ultra phenomenon, until its total eradication.”
Spartak won the second leg 2-1 but Athletic went through 4-3 on aggregate.