The air pollution makes it look dreary, but the revolutionary tunes liven it up. My office elevator is maybe 4m2 and can fot 24 persons or 6 per m2. 880x500 is 440000m2 so lets say 1,760,000 adult communists can fit in that square.
That many people at say 60 kg each with their red books in hand would weigh 105600 tons. I'm no expert but if they jumped with reverent devotion their leaders inspire, that would probably cause a decent sized earth quake.