<<<<The difference between an anarchist approach to organizing a march and typical authoritarian leftist and liberal methods is that the latter seeks to control the actions of the participants, often squashing attempts at direct action. ....>>>>
Consider a small store whose window is broken by anarchists: aren't the anarchists acting as authoritarians vis-a-vis the store-owner? they are dictating to him or her.
A good march would involve democratically-imposed limits on the participants' actions. Obviously, the democracy of a real-world march would be limited (since not everyone is represented on the steering committee, leader are often self-perpetuating, etc.) But it's a mistake to confuse these kinds of limits with those imposed by a small minority (name your least-favorite political activist group which controls a demo from behind the scenes, pushing their Correct Line or favorite politicians). There's a difference between democracy and authoritarianism.
Also, when a small minority of demonstrators (e.g., the Black Bloc) engages in "direct action" -- i.e., conscious efforts to violate existing laws -- this imposes the possibility of being beaten and/or arrested by the cops on the majority of demonstrators who didn't want to engage in "direct action," while likely undermining the stated purpose of the march (which attracted the anti-"direct action" majority to the march in the first place).
Just as with respect to the hypothetical small business-owner mentioned above, the small minority is acting as if it were a dictator, i.e., in an authoritarian way, in the name of their Correct Line (direct-action anarchism). (Unlike with the small business owner, however, the "direct actors" likely profess sympathy with other demonstators. They want to recruit the latter, but not the former, to their cause.)
-- Jim Devine "Segui il tuo corso, e lascia dir le genti." (Go your own way and let people talk.) -- Karl, paraphrasing Dante.