Two Athletic Bilbao security guards were attacked and one was stabbed in the neck by Marseille Ultras on Thursday, say reports in Spain.
Before the start of the Europa League encounter, a group of the visiting fans launched fireworks in the direction of home fans inside the San Mames.
Reports of the stabbing were carried in El Correo, which stated the victim was taken to hospital in the aftermath of the incident, while the condition of the guards is still not disclosed.
Some schools and businesses in Bilbao shut early ahead of the match due to fears over potential violence, with the game kicking off at 7pm local time (CET).
Marseille, who won the tie 5-2 on aggregate, now look set to be sanctioned by UEFA for the disturbances.
Last month, a police offer died after incidents involving Spartak Moscow Ultras in the Basque city.