hy/how do the police have powers to detain such large groups so regularly It's police, after all. They know what to look for.

What does this have to do with travel? Should this question perhaps instead have been posted to the Law Stack Exchange?

It sounds like a quota system. "Come on lads, line up, let's get this over with -- we have to check 20 suspicious persons tonight!" And the "suspicious persons" play along, because it makes everyone's life easier -- if they didn't play along, they might get checked more thoroughly. But this is pure speculation on my part!

@Joe way to confirm that travel questions mean you can ask anything about any country as long as it’s not a country from the majority crowd on stackexchange. Consider how a question ‘what’s up with the US police shooting people on a traffic stop’ would go down.

America where you shut the door and tell the police to leave, rather than solve the issue What are the actions a resident should take against police remaining, with no warrant, on residential property after being repeatedly asked to leave?

@Joe What makes you think that this sort of thing would have been unconstitutional in the US? The fourth amendment? It seems to me that this could easily have been justified as a Terry stop, if the the police had reasonable suspicions (which we don't really know if they had or not). Still an interesting question though.

@jkej Simply stopping people at random is generally not constitutional - for example, stopping people because it's a high crime area or an area with gang activity (as described here) does not justify a Terry stop. The officer has to have reasonable suspicion that this particular person committed a specific crime (or is committing).

Round up the usual suspects

@CobyViner, there is no need to flag multiple highly votes comments to clean up the site, on TSE we seen to have a different attitude to your norm.

@Joe but there is nothing in the question suggesting the people were chosen at random. Simply being in a certain area is not enough to justify a Terry stop, but it can certainly be factored into the totality of circumstances for reasonable suspicion. The reasonable suspicion would be based on location AND the supects behavior in that location. Reasonable suspicion is a pretty low bar for the police to clear.

Depending on when this happened this might also have something to do with the fact that the german police recently shot (and killed!) someone close to the Frankfurt Train Station

@joe Most Germany’s states’ police laws recognize Gefahrengebiete. Simply being at a certain place is sufficient reason for stop and frisk.

Some context: "A Terry stop in the United States allows the police to briefly detain a person based on reasonable suspicion of involvement in criminal activity ... a lower standard than probable cause which is needed for arrest ... search a pedestrian ... a stop and frisk. ... stop an automobile ... a traffic stop. ... the Supreme Court ... define the intersection between policing and the Fourth Amendment protection against unreasonable searches and seizures."

I’m voting to close this question because it is not about travel. Maybe it should be on Politics or Law but this question is not something that affects travellers in particular.

@gerrit psychonoaut It's an issue for travellers, if there happens to be some reason why they might end up in the line-up.

@Araucaria Unlike traffic stops, which specifically target travellers, the police actions in the Bahnhofsviertel don't seem to be targeting travellers in particular, though.

A large number of police making groups of people go certain ways (but usually not lining them up for frisking) on a weekend may happen when police escort soccer fans... Also not so nice for a traveler to end up in between those lines.

@gerrit Do you know that because of the useful information posted here for travellers?