Two television stations and one newspaper said the difference between Andres Manuel Lopez Obrador, the former mayor of Mexico City, and conservative Felipe Caulderon was too narrow to declare a winner until exact results were announced.
The cliffhanger was closely followed in Washington, which hoped to see a reversal of the trend that has brought several leftist leaders to power in Latin America over the past few years.
The parties also waged a close race in parliamentary elections where no party gained an overall majority, according to exit polls.
Two exit polls, both with a 1.5 percent margin of error, gave Caulderon's National Action party 35 per cent, Obrador's Democratic Revolution 31 per cent and the ruling Institutional Revolutionary Party (PRI) 28 per cent of the lower house of the Mexican congress.
The Calderon campaign prayed on fears among Mexican business leaders, likening Obrador to Venezuela's outspoken president Hugo Chavez, and warning he would plunge the country into a ruinous economic crisis.
During his campaign, Obrador accused Calderon of pandering to the rich and promised he would govern for Mexico's 50 million poor.
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.