However, Sunday night, both main candidates claimed victory within minutes of one another in Mexico City, the Los Angeles Times reported.
Mexico has 300 voting districts with 40 million voters, and the top election official, Luis Carlos Ugalde, said there was no clear majority, CNN reported.
"It is not possible within the margins established in the quick count to say what party has obtained the highest vote," Ugalde said.
Conservative Felipe Calderon of the ruling National Action Party and Lopez Obrador, the leftist former Mexico City mayor, both declared themselves the winner Sunday night.
Early Monday morning, with 66 percent of polling stations counted, Calderon's narrow margin over Lopez Obrador had fallen to 1.2 percentage points, the Times said.
In the meantime, outgoing President Vicente Fox called for calm in a nationally televised address.
"The citizens can have the full certainty, the confidence, that all the votes will be counted and respected," Fox said.