Lopez Obrador is calling for a second recount, after losing by one percentage point to conservative Felipe Calderon last Sunday in a vote he claims was plagued by irregularities.
The former maypor of Mexico City plans to mount a legal challenge.
But he was dealt a fresh blow when US President George W Bush and Spanish Prime Minister Jose Luis Rodriguez Zapatero, telephoned Calderon to congragulate him.
European Union observers also said there was no massive vote-rigging or irregularity.
Lopez Obrador has discouraged violence among leftists, many of whom remember a 1988 presidential election widely believed to have been stolen from them.
The left is calling for a vote-for-vote recount, instead of just a new count of vote tally sheets as happened this week, even though Mexican law does not allow for a count of every vote.
The Federal Electoral Institute, which ran the election, said officials from all parties, as well as a million citizens who were called at random to help out on voting day, staffed polling stations and few of them reported problems.
Lopez Obrador was a master of civil resistance in his native state of Tabasco in the 1980s and 1990s when he shut down oil wells and blocked the workings of state government for weeks to protest vote fraud.
Tens of thousands of households in the state still do not pay electricity bills as part of a campaign instigated by Lopez Obrador to complain about a 1994 vote result.
The electoral court has until August 31 to rule on Lopez Obrador's challenges to the vote and until September 6 to formally declare the election winner.