As someone who has acted both as volunteer security and as part of a National Guard anti-riot force, I have a few thoughts on the topic.
First, we've got to get organized. This should be a priority of any group planning a large political gathering.
We need to set up our Security Chief and his Personnel, Command and Communication, and Documentation (i.e. camera men to record any incidents which do happen) teams in advance. We need to establish liaison with the responsible law enforcement agencies ahead of time, so we know what they are planning and who they answer to. (We don't need to trust them with details of our plans, however,). We need to recruit, and train, our security people, and establish what their ‘uniform’ and ‘badges’ will be; what equipment they will be allowed to use (and what equipment we will use if the situation gets out of the control of the authorities, which is not necessarily the same).
On the occasion of the event, we need to establish a Security Command Center, a Personnel Reserve Area, and a perimeter, to separate our event from any ‘counter-protest’. The organizers of the event should insist that the authorities establish a buffer zone between our perimeter and any counter-protest, a buffer zone wide enough that missiles cannot be thrown across it into our crowd. Our “pickets” initially would be posted just inside our perimeter, backed up by camera-men from our security Documentation Team. They would be distinguished by the ‘uniforms’ and ‘badges’ we have decided on, but without visible (or conspicuous) weapons and protective gear. Those weapons and gear would be back at the Security Reserve Area, with the rest of the personnel. (And the even more ‘hard-corps’ weapons and gear would be kept hidden, perhaps just off site, to be brought out if and only if the legal authorities completely lose (or surrender) control of the situation.) The mission of Security would be: initially, establish the perimeter; defend the perimeter against incursion by the counter-protestors; defend themselves and our demonstrators from attacks by the c-pers; disarm the attackers; detain them for the authorities; and do what needs to be done.
It won't be easy, it will be dangerous, but it is going to need to be done.
That’s an ideal. Few groups have either the personnel, training and discipline, or money to afford all the bells and whistles.
Other problems are the “herd of gerbils” you are protecting, who are an unpredictable and often random assembly; as well as agents provocateurs (or agentes provocatrice (female)), who it is now developing were on both sides at Charlottesville, stimulating the confrontation. *And* a hostile government, willing to either pull out the police at the worst moment, or even order the police to attack your group, leaving the enemy unmolested.
There have been instances where comms were cut off by deactivating cell phone towers, or even jammed. They are certainly going to be monitored. This means you may be limited to just hand signals.
Finally, terrorist acts are increasing at an alarming rate, which could easily include snipers and bombs. And of course, an extremely hostile media.