I can answer. Several of us stayed up all night of the election and again on the night of the recount. Just as it was clear on election night that Obrador was going to lose, the police began forming. I think it MAY have been La Enchiladita who was watching Univision that night who saw it. Anyway, I have pinged her to this thread.
I believe it was Wed. night/Thursday morning that Mexican TV showed troops deployed in La Capital, as the final count was underway.
IIRC, your response, Bunny Slippers, was WHOA!