Barcelona and Alaves have both condemned the violence which broke out in Vitoria before Saturday's La Liga match between the two sides.
Reports in the Spanish press claim around 50 people wearing hoods attacked 20 Barcelona fans in the university area of the Basque city in the hours leading up to kickoff at Mendizorroza.
El Correo added that one Barca fan was seriously injured after taking a blow to the head and is currently being treated in hospital.
"FC Barcelona lament and condemn the incidents that occurred this afternoon in the city of Vitoria before the match against Deportivo Alaves," the Catalan club said in a statement.
"The club reiterates its utter rejection of any violent conduct and trusts that the competent authorities will be able to verify the source and responsibility of these events as soon as possible."
While video footage shows that Barcelona fans were the subject of the attack, it has not been confirmed if the aggressors had a link to Alaves.
In the event that they are proven to be connected to the football club, though, the Basque side have promised to take action.
"Deportivo Alaves strongly condemn the violent events that took place on Feb. 11 in the university area of the city between alleged fans of Alaves and FC Barcelona," a statement said.
"The club is available to the authorities and in the case of confirmation that the persons involved are season ticket holders, Deportivo Alaves will assess what measures to take with respect to these people."
On the pitch -- in a dress rehearsal for May's Copa del Rey final -- two goals from Luis Suarez, one each for Ivan Rakitic, Lionel Messi and Neymar and an Alexis Ruano own goal eased Barca to a huge 6-0 win.