This is Lynce.es/ExactCrowd.com, which may still lack the language skills to sell its services into non-Spanish-speaking markets, but provides an excellent stats service to help us assess the claims of various domestic political factions to embody the nation's will in some respect.
Lynce says that forensic counting techniques show what everyone already privately acknowledges - that demonstration organisers routinely lie about attendance numbers. Here out of interest is a comparison of organiser and Lynce numbers for recent demonstrations where both figures are available. Where Lynce provides a higher and lower bound, I've taken the mean.
Averaging out the results, the biggest liars look like the "pro-life" movement, followed by the farmers, trade unions, Catalan separatists, BLT, and so on. Maybe someone would like to add some interpretation. It would be nice, for example, to find in there somewhere there's a desperation factor relating the bigness of the lie to popular indifference to the cause espoused - if anyone was really uncomfortable about abortion, functionaries being put up against the wall, Spain as a unitary state, or whatever, then the parties would have figured it out already from polls, and the soon-to-be-triumphant opposition wouldn't have appointed someone who is widely assumed to be gay as their leader.
The Numbers Man comments: If you go to SIGPAC, locate the Puerta del Sol and pick the area measurement tool you'll find that Sol measures about 7,000 sq. m. If folks are cheek to jowl at 2 sq.ft. per, about 36000 could conceivably fit - no tents, booths, Ford Transits or room to either walk or preach. Give em one third of the maximum plus another 5,000 sq. mt. on the side streets at a concentration inversely proportional to the distance from the centre - and Exact Crowd is probably about right.