BRASILIA, Brazil — Skittish Brazilian authorities on Wednesday spared no effort to boost security in the face of a social media flyer promoting a “mega-protest to retake power” in two dozen cities. Less than 10 protesters showed along Rio de Janeiro’s Copacabana beach; there were more journalists in attendance, plus 29 police vehicles. Citing the call to protest, a Supreme Court justice ordered local authorities in cities across Brazil to prevent protesters from blocking roads or occupying public spaces and buildings.