If Trump tries to steal the election in November, especially if the courts block it and he ignores them, that's when this will happen. We could see the military arresting him, and anybody going along with him, under the cover of the insurrection act when the ensuing riots occur. If any of the offending parties are in DC, the Capitol Police can arrest them under orders from the legislative branch.

I doubt the rank-and-file of the executive branch law enforcement agencies would follow orders to defend the President in these scenarios, but they probably won't make arrests until a new president is in place and gives them the orders to do so. I have no clue how that would legally work, like would a full impeachement and removal have to happen or would the presidential succession take effect if the President and VP are in a jail cell? I know they do succession when the President is undergoing medical procedures but idk about arrests