Electoral officials said they could not release the results of Sunday night's quick count — an eventuality they said would happen only if the leading candidates were within 1 percentage point of each other. Luis Carlos Ugalde, the president of the Federal Electoral Institute, said an official count would begin Wednesday, and a winner will be declared once it's complete.
President Vicente Fox appealed for calm amid fears that a close result would raise the potential for violence. Thousands of Lopez Obrador supporters, waiting for hours in the cold rain in the city's central plaza, began shouting "Fraud! Fraud!" when Ugalde came on live television to announce the delay.
Lopez Obrador said late Sunday that he would respect the delay in declaring a winner, "but I want the Mexican people to know that our figures show we won."
Felipe Calderon spoke minutes later, saying he too will respect the results — but that the official preliminary results, as well as the exit polls, show that he's the winner. "We have no doubt that we have won."
The race exposed deep divisions between Mexico's rich and poor in a nation desperately trying to match the success of its northern neighbor.
Felipe Calderon, 43, of the ruling National Action Party, has been running an exceedingly close race with Andres Manuel Lopez Obrador, 52, of the leftist Democratic Revolution Party. Roberto Madrazo, 53, of the Institutional Revolutionary Party, or PRI, was trailing in third place.
The vote was the first since Vicente Fox's stunning victory six years ago ended 71 years of rule by the PRI, and the six-month campaign was marked by mudslinging and a polarized electorate. The race determined whether Mexico would become the latest Latin American country to move to the left.
Electoral officials said voting was relatively peaceful, although many voters complained that polls opened late or ran out of ballots. Luis Carlos Ugalde, president of Mexico's Federal Electoral Institute, said only eight of the 130,488 polling stations failed to open — the fewest in Mexico's history.
"We've had an exemplary election day, of which all Mexicans can be proud," Ugalde said.
Exit polls indicated National Action did well in three governors races — Morelos, Guanajuato and Jalisco — while Marcelo Ebrard of Lopez Obrador's party easily won the Mexico City mayor's post.
As for Congress — key to determining whether the next president will be able to push through reforms — none of the parties dominated. Two exit polls, both with a 1.5 percent margin of error, gave National Action 35 percent, Democratic Revolution 31 percent and the PRI 28 percent of the lower house of Congress.
Analysts have raised fears that Lopez Obrador supporters would take to the streets in the event of a tight race going to Calderon. A drawn-out period of uncertainty could rock financial markets and unsettle Mexico's maturing democracy.
Carolina Nougue, 35, said she was worried about that possibility. Nougue, a quality control manager for a perfume factory, said she was reluctantly casting her vote for Calderon to keep Lopez Obrador out of office.
She described herself as leftist but worried that Lopez Obrador would govern like radical Venezuelan President Hugo Chavez, and she was turned off by his pledge to put the poor first.
"The division isn't between the rich and the poor. It's between the ignorant people and those who think," she said.
She voted in a polling place tucked between two shopping malls in an upscale Mexico City neighborhood.
Across town in Nezahualcoyotl, a working-class city of 1.2 million on the outskirts of Mexico City where infrastructure hasn't kept up with explosive growth, voting was delayed by flooding from a powerful hail storm the night before. Juana Velasquez, a 63-year-old storyteller, and other residents had to spend the morning bailing water from their homes.
"Every year, it's the same. You just vote for someone who doesn't do anything," said Velasquez, who cast a protest vote for Roberto Campa, of the minor New Alliance Party.
Lopez Obrador, dressed in a brown leather jacket, was the first candidate to cast his ballot, and had to wait nearly an hour before volunteers were ready.
"We did our part," said Lopez Obrador, the candidate of the Democratic Revolution Party. "We are going to wait to see what the people of Mexico decide."
During his campaign, he accused Calderon of catering to the rich and promised he would govern for Mexico's 50 million poor. Many followed his message like a religion, crowning him with flowers at rallies and plastering their cars with his optimistic slogan: "Smile. We are going to win."
I believe he represents hope, especially for people with low salaries," said Armando Juarez, a 46-year-old high school teacher.
Calderon has warned that Lopez Obrador's proposals, including government handouts for the elderly and poor, will bankrupt the nation.
Wearing a suit and tie, he showed his right palm before voting in Mexico City, a reference to his "clean hands" campaign slogan.
"It has been a tense, competitive campaign," he said, adding: "Tomorrow, we have to start a new era of reconciliation."
Casting his ballot in his home city of Villahermosa on the Gulf coast, Madrazo said he was sure he would win, despite trailing third in the polls. "We ask all the candidates to respect the results," he said.
Mexican law limits presidents to one term, and provides for an unusually long handover of power. Fox will remain in office until December, then retire to his ranch.
On Sunday — six years to the day after his historic victory and his 64th birthday — he gave an ink-stained thumbs-up to show he had voted and said: "I know that there are no Mexicans who want to go against democracy."
All three candidates promised to strengthen relations with the United States while opposing increased border security measures unpopular in Mexico.
The estimated 11 million Mexicans living in the United States were allowed to vote from abroad for the first time, but the 32,632 ballots they cast weren't likely to make much of a difference.
Many of those who missed out on the new mail-in vote traveled to Mexico on Sunday to cast their ballots at polling stations set up along the border, including Maria Salome Rodriguez, a 38-year-old farm worker.
She and her husband drove from Fresno, California after their absentee ballots were rejected because of a mistaken address, and waited for two hours to cast their votes in Tijuana.
"We want to vote so Mexico can improve and offer jobs to people here, because even though we're far away, our heart is still with our homeland," Rodriguez said.