Nobody needs to agree on anything. Needs no central planning at all.
Takes about 3% of the population to successfully accomplish something like this.
Basically a couple million people independently decide a line has been crossed, and then they independently pick targets of opportunity.
Basically, imagine a million lone wolf serial killers targeting random supporters of the regime.
They wouldn't all come to the conclusion at once obviously, but when the most hard core ones start to act, the regime's reaction will inevitably be to crack down harder on the public, and that will cross a few more people's lines, etc...
Wondering when one of those 3%ers shows up.