The runner-up in Mexico's presidential election late Monday rejected the "fraudulent" victory of rival Enrique Pena Nieto, raising the specter of protests that rocked Mexico City when he lost six years ago.
When Lopez Obrador lost the 2006 presidential election by less than one percent he claimed fraud, and organized mass protests that virtually paralyzed Mexico City for more than a month.
The first official results from Sunday's vote showed Lopez Obrador from the Party of the Democratic Revolution (PRD) with 31 percent of the vote against 38 percent for Pena Nieto of the Institutional Revolutionary Party (PRI) -- a much wider margin than six years ago.
"We cannot accept a fraudulent result, nobody can accept that," Lopez Obrador said at a press conference, decrying Sunday's vote as a "filthy ... national embarrassment."
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 claimed the PRI, through its national party and governors, spent millions of pesos buying votes. He also charged that the news media heavily favored PRI candidates and that the party shattered campaign spending limits.
"We will provide evidence for these claims and will file appropriate legal action," said Lopez Obrador, emphasizing that he and his supporters will first scrutinize the balloting information with election officials.
He was coy about whether he would call for protests like in 2006. "We're going to wait," he told reporters.
Pena Nieto earlier said today's PRI was a party that respected democracy.
"There is no return to the past. This PRI that is coming into office has proven its democratic conviction," the 45 year-old virtual president told foreign reporters.
It seemed inconceivable 12 years ago that the PRI would be back in power soon, if ever. But after a spiraling drug war with Mexico's powerful cartels in which more than 50,000 people have died during the presidency of Felipe Calderon, from the conservative National Action Party (PAN), handed the PRI a new chance to prove itself.
The economy grew under Calderon, but so did poverty: 47 percent of 112 million Mexicans are poor, according to figures from the government, Latin America's second biggest economy, closely tied to those of the United States and Canada in the North American Free Trade Agreement (NAFTA).
During a call with Pena Nieto to congratulate him, US President Barack Obama "reiterated his commitment to working in partnership with Mexico" and promised to advance "common goals," the White House said in a statement.
Those goals include "promoting democracy, economic prosperity and security in the region and around the globe," the statement released in Washington said.
"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 just after the first official results were out.
Far behind in the initial results was Josefina Vazquez Mota from the unpopular Calderon's PAN party with 25 percent.
Pena Nieto said he will maintain Calderon's unpopular strategy of using the military to attack the drug cartels and capture crime capos, but stressed that he also will focus on bringing down crime.
"Society clearly expects immediate short-term results ... like lower crime and kidnapping rates," Pena Nieto said.
An ex-governor of populous Mexico state, just west of the capital, Pena Nieto is married to glamorous soap opera star Angelica Rivera. He rose to power with help from family connections with powerful old guard PRI politicos and a savvy media team that carefully stage-managed his appearances.
Pena Nieto has promised public works like roads and bridges to increase employment.
Republican US senator John McCain of Arizona, bordering Mexico, tweeted "Congrats to Mexico's new President-elect. Will be interesting to see how he approaches drug trafficking et other issues of mutual concern."
Media projections indicate that the PRI will have a majority in both chambers of Congress. In the Senate, the PAN has the second largest number of legislators, while the PRD is second in the chamber of deputies.