Due to faulty intelligence, the cops were under the impression that the visitors were a ragtag army outfitted with chainsaws, hammers, Molotov cocktails, and even guns. You know, typical hippie stuff. In reality, they were weed-lovers of all ages, including many families, who were trying to throw a free festival without the proper permits. The cops re-rerouted the convoy of buses to a roadblock, then began destroying the vehicles and dragging people out.
And that was just the warm-up. Later, they put on riot gear and began beating people with truncheons and setting cars on fire, and they even killed seven dogs. It's easy to get carried away when you're "in the zone." All those arrested and beaten were then charged with obstruction of justice (as in, their faces obstructed the path of all those batons).
The hippies might have faced criminal proceedings if it wasn't for the unlikely help of a powerful aristocrat. The night before, the convoy had stayed in an area owned by the Earl of Cardigan, who decided to follow the group on a motorbike, probably out of the same curiosity that motivates David Attenborough. He witnessed the full horror of the police brutality, and since rich people matter more than some poor saps, his testimony was key in helping the civilians. For his efforts, the conservative press labeled him a class traitor, but at least he never has to pay for a doobie ever again.