I don’t think arrest is on their minds.
If they have some good solid legal precedent for detainment, then they might try to have their CWII opening move be the mass arrest of would-be resistance leaders (see Using Metadata to Find Paul Revere) — following this arrest they could be moved to a secure facility and 'detained' (killed, but the cover would be detained [possibly keeping them alive long enough to make video-clips that they could release as proof they're still alive])… again, they could be using normal troops for rounding up and only need a relatively small number of those loyal-to-the-cause to operate the secure facility.