As a photographer or journalist you are allowed to document contemporary history (Zeitgeschichte). A bystander at a protest is not a person who falls under that label. A politician is. A protester can sometimes be photographed individually, but in general you should only photograph groups of over five people without a main subject.
A police officer in Germany is not defined as a person of Zeitgeschichte, except when he is actively taking action in a protest for example. You can photograph police officers beating up protestors, but it depends on the circumstances if it's actually legal to show one individual police officer beating up one individual protestor, because the contemporary history usually affects groups, not individuals. They can detain you if they even suspect you have been taking pictures of them.
Still, it's not really legal to photograph people when they are dying on the ground, because that is an emergency situation where you would infringe on the right to dignity. A photograph like >>3762120
might be history now, but in actuality it's a legal minefield to publish such a picture, because it shows a child in an emergency situation.
Coming back to police officers: they are not a person of Zeitgeschichte by itself, if they are going around in a city, but they could be a subject of Zeitgeschichte, if there was a terror atack in that city a day ago, and now they are patrolling in groups with machine pistols. In general publishing such a picture is only allowed, if there is special public interest to be informed. We have a law called "Recht am eigenen Bild" (lit. Right to your own picture) which always comes into play and has to be weighed against the special public interest to be informed f.e..