It’s already happened, with no mass bloodbath: The Black Panthers group marched into the inside of the California State Capitol in Sacramento decades ago, armed to the teeth with rifles, shotguns, and pistols (prior to California rigid gun control laws) and staged a protest on the Senate floor. No one was shot on sight. They also had several protests in other states where they were legally openly-armed and brandishing open-carry weapons. Again, no violence ensued, from either them or the police.
Long time ago indeed, and gun laws in many of these areas have become far more restrictive; but for areas where protesters are complying with legal open carry (areas that have that), then chances of anyone being “shot on sight” by police, are pretty nonexistent.