I am working with the planners for the G8 (and now the G20) that is going to be happening in Canada June 2010.
They have already received intel that these same groups of bussed in protesters will be coming here. Specific plans to accommodate peaceful demonstrations are already being made. There is a lot of resources being put in place to ensure demonstrations can happen.
Of course planning for the 'not-so-paceful' is also happening.
Due to jurisdictional issues it is too large for local cops only, Provincial (state) cops also need to be involved and Federal due to the presence of so many politicos. Then we also have CSIS (Canada's Intelligence Agency) plus law enforcement representatives from each of the attending Countries' secret service.
So I put it out there, if protesters become violent, how should authorities respond? In the presence of violent protests, smoke grenades, pepper spray, the smartest move is to go in, make a quick apprehension, get to a place of safety, then go through the legalities of the arrest. Being surrounded by protesters is not the best time to be reading Miranda to an accused.
Also it should be repeated that fatigues do not necessarily mean "military." I did some training as medical support to a non-military special joint force task force a few years back. My uniform as a medic for this task force was fatigues. Again there was no military involvement on this team.
That said, due to the risk and threats of violence already received, it would be foolish not to include the military as a surge capacity to respond to large scale protests. However they would be acting in a 'deputy to local authorities' role not as the military.
Protesters have every right to protest and authorities need to do as much as possible to ensure this is possible. I think people would be surprised as to how much time is dedicated to ensuring this happens. However once it becomes violent, or results in personal or property damage those rights are suspended until those threats are removed.
If people keep it peaceful, there would not be an issue. There would be no need for arrests, tear gas etc. I put the onus where it belongs. Protestors need to keep it peaceful. If they don't authorities will need to respond (measured and according to law of course).