I live in Alkmaar, was born and raised here, and have never seen this much police in and around the city as I saw yesterday. They were really worried about the fans of Legia.
It's the story of Legia against the story of the Dutch police now. Police here claims that fans of Legia were aggressive, managed to steal pepperspray and batons from the police, and Legia fans without tickets stormed the stadium and managed to get in. The two players that got arrested allegedly abused personnel of AZ Alkmaar.
Polish politicians are now also complaining, contacted the Dutch Prime Minister and even proposed to get the European Parliament involved in this case. Dutch media suggest that it's because of the upcoming elections in Poland as their party, PiS, is known for being critical towards Western Europe in general. Not sure if there's any truth to that.