Feyenoord have condemned a "totally reprehensible" attack on top officials from Europa Conference League rivals Union Berlin at a Rotterdam restaurant.
According to reports in the Netherlands, Union president Dirk Zingler and director Oskar Kosche were among those sitting outside when a group of men began hurling missiles, including glasses and chairs.
Feyenoord said "minor injuries" were sustained by those under assault, declaring those responsible had no right to consider themselves supporters of the Eredivisie club.
In a statement, Feyenoord confirmed the attack on Wednesday evening occurred in the city centre.
"It was a totally reprehensible event that should not happen to anyone who is a guest in the city for what should be a beautiful European football night," Feyenoord said.
"Feyenoord thinks it is terrible that this has happened to representatives of Union Berlin. The club therefore distances itself in every possible way from the people who have reduced themselves to this cowardly act and believes that no one who portrays Feyenoord and the city in such an insane way in a negative light can and should call themselves a supporter. The club cannot get over the fact that people think it is in any way acceptable to display such behaviour."
Feyenoord and Union go head to head on Thursday evening in Group E of the third-tier competition. The teams meet again in Berlin on October.
The Dutch club added: "Feyenoord is of the opinion that the [sporting] battle between two football clubs, in this case Feyenoord and Union Berlin, takes place on the field at all times for 90 minutes and never in any other way, especially not by threatening or injuring. For people who think otherwise, there is zero place at the club.
"Feyenoord also hopes that the perpetrators are found and punished for this shocking case of public violence and although outside its sphere of influence, Feyenoord apologise to Union Berlin for what has happened."
Union responded by quote-tweeting the statement, adding: "Thanks for the clear words Feyenoord."
Local police confirmed they were investigating the disturbance.
Rotterdam-based newspaper Algemeen Dagblad reported a woman needed hospital treatment for a head injury sustained in the attack.
The co-owner of De Huismeester restaurant, Pascal Dijkkamp, described the incident to the newspaper, saying: "They started throwing everything to hand: chairs, glasses, ashtrays. We've already collected a large garbage bag full of shards. A chair is completely written off."