Any mob, no matter how well ordered and intentioned is but one false move away from becoming a riot.
If the cops see a bunch of people coming at them, they will start firing off tear gas canisters, which would create panic.
If, for some reason, that didn't happen, when the protestors got to close, and the cops tried to arrest them, all it would take is for one cop to make one wrong motion, and again, you've got a riot.
the idea is, once it reaches critical mass, it will reach the point that Jeb will have to intervene on the side of the protestors.... And in any case, depending on the numbers out ther gunning for terri, the cops would be incapable of handling it...