Legitimate protest is one thing: rioting is quite another. I’m no supporter of the companies who happily sell stun guns and cattle probes etc to repressive regimes, but today’s invention is certainly non-lethal and might help disperse people in a street riot before anyone gets hurt (they can mock their government online…often more effective and usually less dangerous).
Take a standard polycarbonate riot shield and embed twenty or thirty loudspeaker cones in its surface, facing forwards. Each shield bearer, and that probably means policeman, would be equipped with a belt mounted battery which would drive all of the loudspeaker cones on a shield.
This would generate enough noise to make standing in front of a wall of such shields uncomfortable (even with ears plugged). The shields themselves might be slightly concave so as to help focus the sound energy emerging from them. A large number of such shields could be coordinated by aiming them at e.g. anyone foolish enough to be carrying a weapon. This cacophony could be further intensified by driving the speakers in phase and thus setting up a ‘wall’ of antinodes between crowd and police.