Will fringe left dominate NYC protests?

A political writer for the New York Observer has an article at TNR online about the recent protests in NYC. It's here:

http://www.tnr.com/doc.mhtml?i=express&s=smith032204

I think that the show of anti-Bush strength in NYC during the Republican convention could be a great thing... if it doesn't come off as the cause of extremists. Certain groups show up for every lefty protest and try to make it their own. Frankly, I find this distasteful. If a protest is about opposition to the Iraq War, then stay on that subject. Have a booth about your own cause, hand out pamphlets, but don't try to turn the event toward your own personal cause.

I wonder what the DNC and other Democrats can do to ensure safe, peaceful, on-message protests in August. Or will that be too difficult?