The mayor is the front-runner for next year's presidential election, but contempt of court charges related to a land dispute could legally prevent him from being a candidate.
A congressional committee is expected to vote in coming weeks on whether Lopez Obrador can be stripped of his judicial immunity and tried on the charges.
His supporters say the case is minor and a back-handed way for other political parties to end his candidacy, and the mayor has repeatedly accused President Vicente Fox's conservative government of leading a plot against him.
"Let the man work!" blared crude, hand-written signs tied to overpasses all over the metropolis of some 20 million people. Dozens of the city's trademark green Volkswagen Beetle taxis had slogans written in shoe polish on their rear windshield bearing the mayor's initials: "I support AMLO."
Fox's government responded on Monday night by accusing Lopez Obrador of trying to stir up social unrest.