Do you really think that's what happened in the video you posted? The police arrested a man and held back the other protesters because they were riled up, shocked and probably scared. No one was assaulted, one person tried pushing past the police only to be moved back by minimal force - which didn't stop him instantly crying 'assault' like a toddler.
I think you just have an issue with authority. How do you expect the police to act in that situation? They didn't go there to stop any protests, they went there to arrest someone and that's what they did.