Zinedine Zidane joined Spanish politicians in expressing condolences to the family of the police officer who died following Thursday's clash between supporters of Athletic Bilbao and Spartak Moscow.
Inocencio Alonso Garcia, 51, suffered a fatal cardiac arrest and was pronounced dead at the Basurto University hospital after groups were filmed throwing flares amid violent scenes.
Over 500 officers of the Ertzaintza - the Basque police force - are reported to have been deployed for the Europa League match at San Mames.
Real Madrid coach Zidane extended his condolences at a press conference on Friday.
"We are saddened by the death of the policeman in Bilbao," he said.
"Our thoughts are with his family at this difficult time.
"I hate violence wherever it occurs. It is hard to understand when these things happen. I hope it is the last we see of these kinds of events at a football stadium."
Spanish prime minister Mariano Rajoy was joined by Athletic in offering sympathies on social media, while minister of sport Inigo Mendez de Vigo condemned the behaviour of the "barbarians" involved.