The runner-up in Mexico's presidential vote demanded a full recount of Sunday's balloting, raising fears he could lead disruptive mass protests like he did six years ago.
"There is no doubt that there was not a fair and transparent election," said leftist Andres Manuel Lopez Obrador, accusing the rival Institutional Revolutionary Party (PRI) and its candidate Enrique Pena Nieto of having bought "millions of votes."
Observers fear Lopez Obrador's refusal to concede could trigger a repeat of the 2006 presidential election, when he lost by less than one percent, claimed fraud and organized mass protests that paralysed Mexico City for more than a month.
The first official results from Sunday's vote showed Lopez Obrador with 31 percent of the vote against 38 percent 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.
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 "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 favored 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 "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.
President Felipe Calderon from the right-wing National Action Party (PAN) congratulated Pena Nieto in a phone call late on Sunday. "If these results are confirmed... Pena Nieto will be named president elect, and starting December 1, will be the next president of the republic," Calderon said.
Even though many people will ultimately accept Pena Nieto, there is plenty of anger over the PRI's return to power and about the widespread reports of PRI gifts handed out to sway voters.
There is no run-off vote in Mexican presidential elections, meaning that about 62 percent of the electorate did not vote for Pena Nieto.
Marches are likely to follow in the next days and will tap into that anger, but it is unclear how long they will last, or how many people will follow Lopez Obrador.
Students from the #Yosoy132 movement, which has scrupulously maintained its independence, marched 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 Televisa, and believe networks and the country's big businesses conspired to return the PRI to power.
Students and sympathizers also gathered on Tuesday to protest in the Zocalo, the main Mexico City square.
More than 25 000 protesters in a march organized 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.
In calling for a recount 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 recount call could lead to a split among the left, especially by those PRD legislators who were elected in a vote their leader is now questioning.