IF any violence occurs, it'll be from a tiny minority of the protestors that do NOT represent the vast majority of hard-working, tax-paying, law-abiding undocumented immigrants struggling for fairness and justice against needless punishment and hostility.
The peaceful people marching will be AMERICANS:
Any isolated incidents of violence are NOT representative of their mission or their goals and should NOT be associated in the news with their noble cause.
But if there ARE any gring... er, counterprotesters there tomorrow - they need to be on their best behavior and make extra sure they do not provoke anyone to violent reactions by their words, flags or presence there. If their presence seems to agitate the peaceful marchers, they need to back away immediately and let the lawful march continue.
And if they do happen to be hassled or roughed up by any hooligans tomorrow, they should take note of their location and description of the ruffians and alert the appropriate authorities to file a complaint. Do NOT escalate the violence or you'll be just as guilty as those who start it.