The only problem is that international law doesn't govern how a country conducts activities on their own people. But it reflects a widespread consensus nonetheless.
Note: I'm not accusing the police of terrorism -- that would be pretty massively inflammatory and not my intent -- but simply giving examples to support a widespread discomfort with using non-uniformed forces.
This is not problematic on the face of it -- police are expected to use non-lethal tactics whenever possible, and the laws of war are hugely biased towards killing rather injuring because overwhelming your opponent with casualties is considered inhumane in war.