Aside from the unhappy circumstances which led to Ian Tomlinson's death (and the reason for the OP), from a policing perspective mass demonstrations are incredibly difficult to control - a fact that the organizers often exploit to cause as much conflict as possible with the authorities.
The police response to these demonstrations is necessary for the maintenance of good order and public safety: Oftentimes things go wrong – badly wrong as in this case – but these demonstrations must be controlled, not to do so would be an abrogation of police responsibly and an invitation to anarchy.
So, while condemning the action taken by Constable Harwood, I am mindful of how easily things can run out of control in such circumstances – the trick is to provide an environment where people have the right to demonstrate lawfully and in safety while ensuring that the gathering is not hijacked by any organized group intent on an entirely different agenda.
P.C harwood is a thug in a riot suit...as a officer of the law you have to use reasonable force...how is it D.D reasonable to hit and push some walking away from you..we have all seen the video of the incident and at no point did harwood use reasonable force...i understand policing a violent demo is difficult but how hard is too understand someone walking away from you does not pose a threat.... 
Yes I do agree that it is difficult to understand Constable Harwood's motivation for his chosen course of action - I certainly can't, based on what I saw in the video news reports.
The constable embarked on a course of action for which he will now have to answer in a criminal court.
We have witnessed, in this incident and many other recent incidents, a side of modern policing which dismays me.
I became a police officer to help people, not to hurt them, I wanted to uphold the law, not to break it - it now seems that some police officers aren't particularly concerned about either consideration when in the discharge of their duties.
Make no mistake, policing in today’s society is very difficult - but whatever the extenuating circumstances, the integrity of the law and of those who discharge it is of paramount importance and few circumstances excuse crossing the Rubicon of fairness, impartiality and honesty in how we, as police officers, behave in the course of our duties.