Borussia Dortmund have criticised players, including Emre Can and Erling Haaland, who took part in celebrations with fans outside their training ground after Saturday's 4-0 win at Schalke 04 and called their actions "not tolerable."
The Bundesliga club was hit with a €75,000 ($91,000) fine on Monday by the German Football League (DFL) for breaking the league's coronavirus regulations.
"It's out of question that the Revierderby is a special match. In spite of that, the club is responsible to control the compliance with the regulations," the DFL said in a statement and referred to the league's hygiene protocol agreed to by all 36 clubs of the upper tiers of German football.
As Dortmund returned from their game on Satureday, they were met by approximately 200 fans and joined in with chants and songs from inside the team bus as well as filming the scenes outside.
Footage posted on social media showed that the players did not wear masks and did not maintain social distancing as required in the DFL hygiene protocol.
Police and club stewards did not intervene as fans stopped the bus, with some of the supporters not wearing masks and the majority not maintaining social distancing.
The German Football League (DFL) have demanded an explanation from Dortmund after large unorganised groups greeted the team bus with flares at their training ground after the win.
The club said in a statement on Sunday those celebrations were "not tolerable."
They added they can "only apologise and promise to address the question how to completely prevent these scenes in the public in the future with police and all involved at short notice."
Under the DFL hygiene protocol, the German Football Association (DFB) can punish breaches of the protocol inside the stadiums and the clubs are responsible for those occurring in private situations.
"The DFL has instantly contacted Borussia Dortmund and asked the club for a statement at short notice," a DFL spokesperson told ESPN on Sunday.
Dortmund are the first club fined for a breach of the protocol this season.
The fine will not be imposed on Dortmund for the behaviour of their fans, only for the breach of the COVID-19 rules inside the team bus.