are police the law or do they uphold it?

today i was convicted of obstructing a police officer that i was trying to walk around, as he followed me to push me back with his shield. i think most people would accept that i was being obstructed by him. the difference comes when you realise that i was walking towards a power station which apparently makes it alright. he said the police had established a 'sterile cordon' which people weren't allowed into but couldn't say which law had been used to establish this - the reason - there was none. police think they are in a war film 'take your men and set up a perimeter over there' they are ordered 'and don't let anybody through' and off they go and do it. the law doesn't matter, they are the law so whatever they want to do is ok. when i arrive i'm told to stop, in court he said 'most people stop when i ask them, they don't need to ask what my legal basis is for doing this' maybe they should? he admitted he had no idea what law he was using or even who's decision it was that people weren't allowed in to that area. but he was happy to repeatedly push me with his shield and eventually wrestle me to the ground and pin me down with his hand around my throat. this is the tactic that forms the basis of kettling - restricting people arbitrarily just because they are police and we are not, deciding what people can do without telling them or referring to the law.