Two leading newspapers' exit polls said Felipe Calderon of the conservative ruling party was essentially neck and neck with Andres Manuel Lopez Obrador, the left-wing former mayor of Mexico City.
Electoral officials said they would continue counting votes and hopefully declare a winner later Sunday, but they warned they might hold off -- even for days -- if neither candidate had a large enough lead, reported the Associated Press.
The extremely tight vote raised fears that either of the main candidates may challenge the results and call for street protests.
Meanwhile, Roberto Madrazo of the Institutional Revolutionary Party, which ruled Mexico for 71 years before Vicente Fox won the presidential election in 2000, appeared to be trailing in third place. His aides rejected the poll results and said they were waiting for an official tally, according to Reuters.
Lopez Obrador appeared to be the favorite for much of the campaign, but aggressive television advertisements from his competitor painted him as a danger to Mexico's economic stability.
He has been compared to Venezuela's anti-U.S. leader Hugo Chavez.
Calderon, a Harvard-educated lawyer and economist, has promised to create millions of jobs with pro-business reforms.
When Fox, a member of the same National Action Party as Calderon, became president after pledging economic reforms, he ran into problems with opposition parties in Congress, which blocked his promised reforms.
Fox was unable to run again under Mexico's constitution.
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