The coordination is the thing here, that's many units being instructed to carry through in the same manner.

As for the numbers:

  Interior Ministry reports say security forces confronted demonstrators in more than 400 cities and towns, with more than 4,000 clash locations reported nationwide

As to the how, the article suggests some deaths immediately occurred in crowds - firing, dispersing, funneling, crush injuries, etc. leading to many intakes to hospitals and treatment tents etc ... followed by execution of the injured.

It's grim stuff.

Some years past the waves of the Rwanda massacres saw almost a million people killed in bursts across 100 days, mostly with machetes and hand guns.

The numbers reported here are absolutely feasible, the reporting is certainly questionable; bad things happened, but was it at the claimed scale?