Government officials said some 1.2 million backers of Mr Obrador filled the capital's streets chanting "no to fraud" and calling for a vote-by-vote recount of the 41.7 million ballots cast in the election.
The rally was the largest for Mr Obrador since the election.
Mr Obrador also asked his supporters to set up 47 camping sites across the gridlocked city's main thoroughfares that would entertain residents with art and performances while peacefully lobbying for the losing candidate's cause.
"I told them I myself will be living in one of those sites while we are holding these meetings," the popular former Mexico City mayor said.
"I know that what I propose is not easy, but it is essential for our cause."
Conservative Felipe Calderon defeated Mr Obrador in the polls by a bare 0.58 percentage point margin.
On July 18 Mr Obrador’s Party of the Democratic Revolution sent an 836-page document to the elections court claiming that the vote was invalid.
They have also supplied videos and other alleged evidence of cheating to back their claim.
The elections court, which has the power to call a partial or a full vote recount, must declare the official victor in the poll by September 6.
The winner will assume the presidency on December 1.
Lopez Obrador maintains that if the commission does not order a full recount it will be covering up a "general election fraud" to Mr Calderon's benefit.
For their part, Mr Calderon's supporters in the National Action Party branded Mr Obrador's statements as "schizophrenic", and insisted that Mr Calderon has not deceived Mexico's people.
"It is not possible, it is not necessary to deceive people about anything using claims and suspicions. Personal beliefs must be put aside for a course through institutional channels established by law," Mr Calderon said.
On July 16, Mr Obrador drew 800,000 people out to protest in his support across the country, and has since called for a campaign of "peaceful resistance".