1. that the police used force at all
2. that the civilian on which force was used did something to justify it
For claim 1, the police absolutely have the presumption of innocence. But once they concede that they did use force, the burden of proof flips around. Otherwise, you're placing a presumption of guilt on the civilian by asking them to prove that they weren't threatening a cop.
This is essentially an affirmative defense [0]: it concedes the unlawful action (e.g. police use of force) but alleges a specific set of circumstances that would shield the person from repercussions. The burden of proof lies with the one using the affirmative defense.
> that the civilian on which force was used did something to justify it
When was that proven like you demanded?
> I never said anything about self–defense.
If it isn't self-defense, then they just illegally assaulted one or more officers and deserve to be punished.
> If they are unjustified, then you should be able to prove that. Otherwise what you're talking about is the police having to prove their innocence.
If I allege that a police officer used force on me, they have the presumption of innocence. However, if they admit to using force, that presumption vanishes. We can assume they did it, because they just admitted to it!
The question then becomes whether or not that force was justified. In answering that question, the burden of proof shifts to them: I am presumed innocent of threatening them, and they must affirmatively prove that I did something to justify their use of force.
Edit: I never said anything about self–defense because your original comment was about the burden of proof for use of force by the police, not against them. You are moving the goalposts.
But for some reason the moment the police are involved, that presumption is reversed. Why?
When did the tear gas throwers show they were justified in throwing tear gas at the police?
Throwing tear gas back (important word) at the police when you feel it’s unjustified is an interesting question. Arguably such an act could be an act of self defense, but that gets very complicated when the police are involved.
What right do citizens have to defend themselves against what they feel is unlawful police violence? Against non-state actors, you have the right of self defense, obviously. Against state actors it is obviously complicated, since nominally they have a monopoly on the legal use of force, and we don’t want everyone with a valid warrant to feel justified in shooting at the police.
But to assume that there is no force that the state can apply that would not justify self defense is to accept a complete erosion of all civil liberties once the police are involved. Not all wrongs perpetuated by the police are amenable to court restitution; if you lose out on your right to peacefully assemble, then can a court give that back after the fact? What happens if the police apply lethal force without justification; you can’t sue to get your life back.
It is obvious that some right to self defense against state action exists, but exactly where the line is seems quite fuzzy to me.
Because the testimony by police is treated as a proof more reliable than nearly any contrary evidence, short of very clear video.