A Basque police officer has died after suffering a cardiac arrest following clashes between Athletic Bilbao and Spartak Moscow fans before their Europa League tie on Thursday night.
The Spanish Football Federation named the police officer as Inocencio Arias Garcia, who suffered a heart attack after a flare was thrown in his direction during clashes between rival fans at San Mames.
Officials have confirmed that Garcia was not in a direct confrontation with any fans when he collapsed.
A minute's silence will be held before all matches in Spain this weekend in memory of Garcia.
More than 500 police officers were deployed in the city for the round-of-32 second leg, with a large number of visiting supporters expected at the match.
"We can confirm that a Basque policeman died Thursday," said a press spokesman for the Ertzaintza regional police said.
"UEFA strongly condemns the violent clashes which occurred in Bilbao," European football's governing body added in a statement. "We are in contact with local authorities to obtain further information on these incidents."
Spanish Prime Minister Mariano Rajoy and Bilbao coach Jose Angel Ziganda offered their condolences to the family and friends of the officer who died.
"First of all I want to give my deepest condolences to the family of the deceased officer," Ziganda said. "I suppose that this needs a reflection, this must be eradicated.
"I don't know when it was or how it was. We didn't know anything. We found out when the game is over. I do not know if it was better or worse to suspend the game."
Basque police had made a number of arrests earlier in the evening after Spartak fans clashed with the authorities as well as rival supporters, with fireworks and other projectiles thrown.
"Five people have been arrested. We do not know if they are all supporters of Spartak, but many of them are Russians," the spokesman for Ertzaintza added.
Spartak won the match 2-1 but Bilbao progressed to the last 16 of the Europa League courtesy of a 4-3 aggregate win.