Katrina is the example that gives me the most hope where this subject is concerned. New Orleans is a comparatively small city, geographically confined between water barriers (which had, at the time, intruded and flooded more than half of the city), and yet despite having National Guard units and volunteer police cars for miles (no exaggeration, I saw it up close), it was a struggle to "secure" the area.
Martial Law may be viable in small areas where the people are frightened and view the action as protective, but on a regional or national scale, where the people are angry and armed? Nope, it's just not sustainable.
Completely unsustainable. It would collapse on many fronts.