To cheers, whistles and shouts of "Fraud, fraud", Lopez Obrador told the crowd that the official razor-thin election victory of the conservative ruling party's Felipe Calderon last Sunday was bogus.
He said he was the legitimate winner and called for more protest marches as his legal team challenges the results before Mexico's highest election court.
Some of the 130,000 people who came out in support of the popular former mayor of Mexico City held posters reading "No to the damn fraud", accused the ruling party of vote-rigging and some hinted of unrest. "If there's no solution, there'll be a revolution," they chanted.
A recount showed that Lopez Obrador lost to Calderon by less than one percentage point.
Lopez Obrador has yet to produce evidence of fraud, and a team of European Union observers has said there was no large-scale irregularity or vote-rigging.
Still, many of his followers are convinced that he was robbed of victory.
More than 100,000 rally at the
Zocalo Square in Mexico City
"First there was one result and later on there was another. Of course there was fraud," said Alberto Gallegos, 33, a chemical engineer. "It makes me mad."
Mexico's left still remembers a 1988 election widely believed to have been stolen from them by the PRI.
Lopez Obrador said his protests would be peaceful, but he would not give in easily. "This is only just beginning," he said, calling for more marches and rallies next week.
The left is calling for a vote-for-vote recount, instead of a new count of polling station tally sheets as happened last week. Mexican law does not allow for a count of every vote.
The electoral court has until August 31 to rule on the challenge and until September 6 to declare the election winner.