@djoerd I'm not dutch, but I know a person in Utrecht.
For what I've been told the protest was interrupted as it was not authorized.
I'm not here to say that it is good to stop unauthorized protests with violence, but why would you not ask for authorization?
It is important to be well organized to perform a protest and it is fundamental to have a security process to prevent infiltrations.
Not having the authorization just confirms way too little organization.
This stuff is actually important, I remember some protest for Palestine a few years ago in which one guy took out one gun and started shooting to disperse people. Since there was a good organization that led to no problems and I believe they were able to escort the guy to the police without anyone getting hurt.
@rastinza thanks for sharing your indirect experiences.