Nothing in this world is a simple matter of black of white; there are many shades of grey in between, especially when it comes to how large demonstrations are policed and even more so when there is resultant disorder which is splashed across the TV and the Press subsequent to the events of the G20 protests.
While you can (and should) ask searching questions about the tactics used by the Police at large demos, and I can accept that in heated situations a Police officer might be less than polite when he or she asks people to move out of the way or along. As an aside, it is worth noting that press photographers have lived with this sort of thing for years, but, there is no excuse for excessive force or violence.
One thing to remember is that there are those who attend demos, who systematically direct what can be best described as targeted hostility towards Police officers. There are those who are looking to provoke a tangible public reaction for their own purposes, and who are more than happy to hide behind more naive and inexperienced demonstrators at large demos.
Now Police officers are effectively empowered to use reasonable force providing that it is proportionate to the situation they find themselves in, if excessive force is used by either side then it should be fully investigated. Any Police officers who cross the line when it comes to acceptable standards of behavior will be dealt with; as should any demonstrators who likewise may have crossed the line of acceptable behaviour.
It is important for us to develop and retain a sense of perspective, we should remember that the vast majority of Police Officers and the vast majority of demonstrators did not cross the line when it comes to acceptable standards of behaviour at the G20 protests, only a small minority did so.
At the end of the day we are dealing with Policing by consent within a democratic society and it is important to remember that Police officers are only human and may react, as would anyone who has been subject to systematic provocation at the hands of a very small minority hell bent on inciting or causing trouble by provoking a reaction from harassed Police officers.
Now it works both ways because the public would expect overly thuggish or aggressive violent behaviour and the use of excessive force towards peaceful demonstrators and similar behaviour from demonstrators towards our Police officers, to be dealt with. Violent conduct from public servants to the public and from the public towards public servants is not, has never been and never will be acceptable and should be subject to the full penalties and rigours of the law - not trial by media.