From the outside it looks like these protests ended up like the "Yellow vests" ones in France: There is valid concern but the movement is taken over by organised, violent extremists.

This brings the question: If violent actions are organised through e.g. Telegram, do the local authorities have a quick way to disable the service?

This seems to be an important question these days, not only in China/HK, but everywhere and we've seen authorities in several countries taking this sort of steps, which is legitimate in some circumstances.