Andres Manuel Lopez Obrador, the runner-up in Mexico's presidential election has demanded, a full recount of the ballots, pointing to "inconsistencies."
"There is no doubt that there was not a fair and transparent election," said Lopez Obrador on Tuesday, accusing the rival Institutional Revolutionary Party (PRI) and its candidate Enrique Pena Nieto of having bought "millions of votes."
Observers fear that leftist candidate Lopez Obrador's refusal to concede could trigger a repeat of the 2006 presidential election, when he lost by less than one per cent, claimed fraud and organised mass protests that paralysed Mexico City for more than a month.
The first official results from Sunday's vote showed Lopez Obrador with 31 per cent of the vote against 38 per cent for Pena Nieto of the PRI - a difference of three million votes, election officials say, a much wider margin in the last presidential election.
Final official results are due out by Sunday.
Al Jazeera's Adam Raney, reporting from Mexico says Lopez Obrador has accused Pena Nieto of buying votes, raising illegal funds and overspending on the campaign.
"It is unclear if Lopez Obrador will get as much support as he did then (in 2006 when) hundreds of thousands of people joined him in blocking Mexico's main avenue for six weeks in their push for a recount," our correspondent says.
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Ricardo Monreal, who coordinated the leftist coalition campaign led by the Party of the Democratic Revolution (PRD), told reporters alongside Lopez Obrador that the coalition formally asked election authorities for a recount "in the 143,000 ballot boxes installed for the presidential elections."
Monreal explained that an "investigation" by the leftist coalition had so far found that 113,855 out of the 143,000 ballot boxes "that were installed had inconsistencies."
Lopez Obrador claims that the PRI, through its national party and governors, spent millions of pesos buying votes. He also charged that the news media heavily favoured the PRI and that the party shattered campaign spending limits.
The PRI was synonymous with the Mexican state as it governed for seven decades until 2000 using a mixture of pervasive patronage, selective repression, rigged elections and widespread bribery.
Lopez Obrador on Monday decried the results as "a national embarrassment" and "fraudulent," but was coy about whether he would call for protests like in 2006.
Officials with the Federal Electoral Institute (IFE), which organised the vote, said on Tuesday that the results of one-third of all polling stations could be subject to review.
Students from the #Yosoy132 movement marched on Tuesday in front of Televisa, which together with TV Azteca form the country's television broadcast duopoly.
The students are angry over the gushing coverage Pena Nieto received from the networks, and believe the networks and the country's big businesses worked together for the PRI's return.
Students and sympathisers also gathered on Tuesday to protest in the Zocalo, the main Mexico City square.
More than 25,000 protesters in a march organised by the #Yosoy132 movement marched late Monday against what they also described as "fraud" in Sunday's vote.
More marches are planned for the next days.
Lopez Obrador is "wasting his political capital," said Javier Oliva, a researcher at the Autonomous University of Mexico (UNAM).
"He agreed to play under these rules, but when he loses he now wants to leave the game," Oliva said.
The move could lead to a split among the left, especially by those PRD legislators who were elected in a vote their leader is now questioning.