Rock-throwing football fans turned Brøndby Stadium into what police referred to as a 'war zone' after the FCK-Brøndby game on Sunday.
In response, Copenhagen Vestegn Police arrested 484 fans at once, many of whom had nothing to do with the turmoil.
But those preventive arrests are illegal and the police may end up having to pay millions of kroner in compensation, according to lawyer Christian Dahlager.
"Based on what I’ve seen so far, the mass arrests following the FCK-Brøndby game on Sunday are illegal," he told BT tabloid. "It puzzles me that Copenhagen Vestegn Police don't understand that it isn't legal to make these preventive arrests."
Similar to COP 15 arrests
In 2010, Dahlager was one of five lawyers who won a case involving the mass arrests of almost 1,000 protesters at the 2009 UN COP15 climate conference in Copenhagen.
In the weeks leading up to the climate conference, parliament passed the controversial law ‘Lømmelpakken’ which allowed police to arrest large groups of people if they suspected unrest to be looming. Both the Copenhagen City Court and Eastern High Court ruled the arrests illegal because they conflicted with the European Human Rights Convention.
The COP15 protesters each received between 5,000-9,000 kroner in compensation and police may have to pay similar amounts to many of the football fans they arrested on Sunday.
"It sounds like the police are well aware that they made a lot more arrests than those who were causing the trouble. You need reason to believe that people have committed a crime before they can be arrested," Dahlager said.
Football clubs invite fans to peace meeting
When the two rival football clubs in Copenhagen meet each other on the field, violence and disorder often follow. To avoid future clashes between the fans and police, FC Copenhagen and Brøndby IF are arranging 'peace meetings'.
"We are going to bridge the gap between the two clubs," the head of Brøndby IF, Tommy Håkansson, wrote in a press release. "I personally think the key is dialogue and closer co-operation across clubs and authorities."
Håkansson and the head of FC Copenhagen, Anders Hørsholt, have called for the first peace meeting to take place next year.
"One of the first questions in these situations is always: 'Who is responsible for the trouble?'" Håkansson said. "Here we all have to be more self-critical. Clubs, fans and authorities alike."