I think this is worse though than just cops getting out of hand.
The Guardia Civil have been around for hundreds of years, and this festival has gone on for a few years without people getting any grief from them.
What appears to have prompted the authorities to act was a change in power in the local council shifting to the right wing. Someone from that council ordered them to break up the gathering.
IMO the people who went to the ballot box and voted for the right-wing candidate, knowing (or at least accepting) stuff like this might happen are as guilty as the cops who swung the batons.
Incidentally Spain had an extremely authoritarian Government (by European standards) based on right up until the late 1970s.
