I shopped in a couple of markets close by. I believe I saw more indigenous racial influemce than Mestizos in the area and few if any Gueros. The indigenous are of course shorter and they clustered apart even though they were the majority attending the "rally". The guy with the bullhorn was a light-skinned Mestizo. The young men with the signs sure looked to be Tzotzil or Tzeltal, from Chiapas.
Because the sign bearers were less than 100 meters from the "rally" it is reasonable to assume the two were connected, vamos a ver.