Edited later to add:
The original press release seems to be gone now, but here is what is claimed to be the revised release, and an article on the same topic.
https://www1.wdr.de/nachrichten/polizei-dortmund-greta-gewaltbereit-100.html
https://www.yahoo.com/news/german-politician-calls-greta-thunberg-102611513.html
Banning a protest under whatever pretext they can come up with is definitely an authoritarian move. Giving police power to do that is not what a democracy should be doing.