Well, if protestors are engaging in illegal activities, they should be stopped. [0]

And putting on masks to obscure your face is illegal in protests in Germany.

See https://de.wikipedia.org/wiki/Vermummungsverbot

[0] I mean that in a legal sense, that's how the police works. Whether stopping the protest is morally the right thing to do or not is a different issue that I don't want to address here.