Mexican left-wing presidential candidate Andres Manuel Lopez Obrador has demanded a vote-by-vote recount, after early results showed he had lost.
Conservative candidate Felipe Calderon has claimed victory by just over 1% in Sunday's election.
Although a formal recount will begin on Wednesday, electoral authorities will only be required to re-check tallies from each ballot box.
Mr Lopez Obrador says recounting each vote would clear up all doubts.
His advisors have threatened to call street demonstrations if legal challenges from their party - the Party of the Democratic Revolution (PRD) - are not considered properly.
Inconsistencies
"We propose that all of them are counted, all the votes from the beginning," Horacio Duarte, of the PRD, told Reuters news agency.
"With such a close result, counting just part of the votes could cast a shadow of doubt.
"It is better to take a couple more days so there is certainty in the country rather than have months of uncertainty."
The preliminary results issued by the Federal Electoral Institute (IFE) gave Mr Calderon 36.38% of the vote compared with 35.34% for Mr Lopez Obrador.
But Mr Lopez Obrador said he would not accept the results, which he believed had "many inconsistencies".
His party claimed some voting places were counted twice while others were not counted at all.
If Mr Calderon's win is confirmed, the victory will halt the rise of the so-called left in Latin America, and the US will continue to have a like-minded administration on its southern border, the BBC's Duncan Kennedy in Mexico City said.
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