Hundreds of thousands of people have joined the march across Mexico City on Sunday to Zocalo, its main square, where Lopez Obrador announced details of a civil disobedience campaign to push the case for a recount.
Mexico has faced political chaos since the election on July 2, which saw Obrador, the former mayor of Mexico City, beaten by Felipe Calderon of the ruling conservative party by just around 244,000 votes out of 41 million cast.
During the rally Obrador told his supporters to occupy Zocalo Square and the city's main roads until a recount was held.
"I propose we stay here permanently until the court resolves this. ... that we stay here day and night," Obrador told the rally.
Lopez Obrador, who campaigned on promises to help Mexico's poor with ambitious welfare and infrastructure programmes, is also challenging the result before Mexico's highest electoral court.
He says he will only accept the result if there is a recount.
While stressing his protests will stay peaceful, Lopez Obrador upped the ante last week by declaring he was the country's legitimate president and warning that his supporters had plenty of energy for more protests.
The protest on Sunday will be Lopez Obrador's third since the election, and could be the biggest.
"We are working intensely. It's going to be a historic march, supporters are coming from all over the country," said Jesus Ortega, a senior aide to the leftist candidate.
Despite growing tension, analysts expect protests to remain peaceful as Lopez Obrador's Party of the Democratic Revolution, or PRD, waits for the electoral court to make its call on allegations of vote-rigging over the next few weeks.
Lopez Obrador claims that vote
counts were fraudulent
"The PRD is worried about the violence it could generate and is trying to avoid it. I don't see violent organisations among Lopez Obrador's backers," said political analyst Carlos Sirvent, referring to Sunday's march.
However large the protest, it is unlikely to directly influence the seven electoral court judges who have until August 31 to decide whether there is a case to reopen ballot boxes.
Lopez Obrador claims that vote counts were fiddled at more than half of the country's 130,000 polling stations.
The judges' choices range from throwing out Lopez Obrador's case and declaring Calderon the winner, to ordering a partial or full recount or even annulling the election and calling for a repeat.
An annulment is thought highly unlikely and, without it, the court must formally declare Mexico's president-elect by September 6.
Calderon insists that the vote was clean and that no recount is needed.
While his party's lawyers are fighting the PRD at the electoral court, he is trying to pull support from other opposition parties for reforms that he plans to push through once he takes office in December.