Hello I am a protester in Hong Kong.
Many protesters use telegram group. I have heard recently from the News that a group admin was tricked by the police to unlock their phone (police cannot force one to unlock their phone under the law of HK) and a list of 20k - 30k participants were exposed. (That group admin was detained by the police as an organizer.)
Personally, I know telegram was never a good choice because it uses home-grown crypto (I don't think a typical user is aware of this). However, I don't think it matters in this case.
How to prevent this from happening? Does using signal group with sealed sender help? Do we need anonymous messaging?
Source (In English): Student arrested for being admin of social media group supporting Hong Kong protests