"'Come on, ladies and gentlemen. Is this any way to behave? You can't take … the law … into your own … hands …' His voice faltered. Sometimes it takes the brain a little while to catch up with the mouth. Vimes turned and looked at the squad, who'd needed no prompting at all to hang back. And then he turned to look at the barricade. Where, exactly, was the law? Right now? […] Where was the law? There was the barricade. Who was it protecting from what? The city was run by a madman and his shadowy chums, so where was the law?
Coppers liked to say that people shouldn’t take the law into their own hands, and they thought they knew what they meant. They were thinking about the normal times, and men who went round to sort out a neighbour with a club because his dog had crapped once too often on their doorstep. But at times like this, who did the law belong to? If it shouldn’t be in the hands of people, where the hell should it be?"
Night Watch, by Terry Pratchett
actually though everyone just needs to read this book ok