On my own I could only translate the word on the left, “狗官”. The first character means “dog”, the second one means “officer” (that one is the same as in “警官”, which means “police officer”).
I wouldn’t jump to a conclusion as to who was behind this—seems about the type of thing one would do to provoke military intervention.
Protesters do half-heartedly distance themselves from this sort of behaviour, but this instance might warrant a louder gesture from their side. Sadly, they’re way too disorganised to bother (or even to be capable of) issuing such a statement over a mainstream channel.
Lack of organisation makes the whole movement accountable for actions of anyone wearing a black shirt, and thus is very easy to undermine, purposefully or accidentally.
Can anyone translate the slogans tagged on the wall?