Update. In my previous post I linked to June 2020 advice from #AmnestyInternational on how to protect your phone from police searches when you attend a protest.
https://www.amnesty.org/en/latest/campaigns/2020/06/tactics-to-secure-phone-before-a-protest/
For something more recent, see this Nov 2024 advice from @eff.
https://ssd.eff.org/module/attending-protest