Athletic Bilbao manager Ernesto Valverde censured the club's own fans and branded them a 'stain on the club' after some lit flares in a 1-1 Europa League draw at Roma.
The Spanish side could face UEFA sanctions after supporters set off the pyrotechnics on their return to Europe for the first time in six years.
In a statement Athletic Bilbao claimed that some fans even launched flares into the home supporters' section.
Senior stars Inaki Williams and Oscar de Marcos marched over to fans to remonstrate with them at full time and tell them it was 'unacceptable'.
'Throwing a flare at opposing fans is a stain on the match,' said former Barcelona manager Valverde.
Athletic Bilbao boss Ernesto Valverde slammed fans after they lit flares while away at Roma
Supporters allegedly tossed flares into the home supporters' section during the 1-1 draw
Valverde decried the fans as a 'stain on the club', which could now face UEFA sanctions
Inaki Williams and Oscar de Marcos confronted fans at full time for their 'unacceptable' antics
'We have not been in Europe for a long time, we have come to play an important match.
'We regret the incidents at the end by a part of our fans who do not represent the bulk of them.
It was a decent performance on the pitch for the Basque side, who went behind to an Artem Dobvyk goal but equalised late on thanks to Aitor Paredes.
Considering that Roma have reached the semi-finals of the Europa League in three of the last four seasons - including the final in 2022-23 - a point away from home is commendable.
However, the club found themselves compelled to release a statement chastising their fanbase after the off-pitch events that marred the evening.
'Athletic Club wishes to express its strongest condemnation of the unacceptable acts committed by a minority of people who travelled to the Europa League match against AS Roma and were located in the visiting stands,' the statement began.
'During the match, this small group lit flares and threw some of them into the home crowd, violating safety regulations and tarnishing the image of our fans, known for their exemplary behaviour and unconditional support for the team in any circumstance.
'These attitudes do not represent Athletic Club's historical values, such as respect, sportsmanship and coexistence.'
The club released a statement promising the 'eradicate' the behaviour with 'zero tolerance'
A late Aitor Paredes equaliser ensured Athletic Biblao a point on their return to Europe
It continued: 'As Valverde said at the end of the match: "For all of us, football is a party. And this is not football, we don't like it."
'Athletic Club, in collaboration with the relevant authorities, is already taking the necessary measures to identify those responsible and ensure that the corresponding sanctions are applied. The Club will work to eradicate this behaviour, with zero tolerance.
'This minority does not represent our fans. Captain Oscar de Marcos, together with Iñaki Williams, made it known to this minority of fans at the end of the match that their attitude “is unacceptable”.'
The club also thanked well-behaved fans and apologised to Roma and their supporters.