IANAL, but I have given the question some thought.
Arkansas is a "duty to retreat" state, with the exception of a Castle Doctrine coverage for domicile and curtilage, but not for business or vehicle (Branca,
Law of Self Defense, 3rd ed., Table 5-1, p. 218).
I presume this means if the maliciously driven vehicle is driving
at you, you have to step out of the way if at all possible. Once the vehicle has passed you, you would no longer be defending yourself, unless it turned and came at you again.
There is an exception in the law for LEOs on that point, but a recent local case has gotten really ugly on that point. Websearch on "
Sharkey, Little Rock, 2019, police shooting" for numerous stories on the case of LEO who did not step out of the way before shooting.
As for defense of others, AR has no conditions different from self defense (Branca,
Law of Self Defense, 3rd ed., Table 7-1, p. 235). I interpret this to mean that the others who are in danger should also be moving out of the vehicle's path (retreating). If they can and do not, they would not be entitled to a self-defense claim, and therefore, you, as a 3rd party defender-of-others would not be entitled to claim a defense of others shooting.
Of course, the apparent speed of the vehicle and proximity to targeted pedestrians would become part of the messy discussion in court, should it come to that. There is a big difference between a car moving slowly though the crowd, possibly bumping into protesters who refuse to move, as the car is trying to exit the area, as compared to the high speed attack that happened in Charlottesville.
Related, I note that I have heard at least three instructors, Andrew Branca, John Correia, and Ed Monk, discuss the decision on defense of others, pointing out that no citizen, armed or unarmed, has a legal obligation to engage in defense of others. It is up to the individual to decide whether there is a moral obligation to do so. Further, all three recommended balancing that against your moral obligations for your own safety and the probable aftermath as it affects you and your family.
Lastly, I think the application of Rule 4 and the likely totally unsafe backstop in a protester crowd situation would mean any attempt to shoot the attacking vehicle or its driver would put many others on the far side of the vehicle in serious danger.
Yeah, I think I would most likely just act like a shepherd and get the flock out of there.