SAO PAULO — Thousands of Brazilians flocked to a law school Thursday in defense of the nation’s democratic institutions, an event that carried echoes of a gathering nearly 45 years ago when citizens joined together at the same site to denounce a brutal military dictatorship. In 1977, the masses poured into the University of Sao Paulo’s law school to listen to a reading of “A Letter to Brazilians,” a manifesto calling for a prompt return of the rule of law.