The runner-up in Mexico's presidential election has rejected Enrique Pena Nieto's "fraudulent" victory, raising the spectre of protests that rocked Mexico City when he lost six years ago.

When Leftist candidate Andres Manuel Lopez Obrador lost the 2006 presidential election by less than one percent he claimed fraud and organized mass protests that paralyzed 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 Institutional Revolutionary Party (PRI)—a much wider margin than six years ago.

"We cannot accept a fraudulent result, nobody can accept that," Lopez Obrador, of the Party of the Democratic Revolution (PRD) 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 the PRI 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 results with election officials.

He was coy about whether he would call for protests like in 2006, saying: "We're going to wait."

Students from the #Yosoy132 movement, however, did not wait. To the cry of "A Mexico without the PRI!" they expressed their anger in a Mexico City march over what they also described as "fraud" in Sunday's vote.

City police said more than 25,000 protesters took to the streets in anger in the city's upscale Polanco neighborhood.

One of the protesters, 20-year-old Bruno Rebolledo, said the protest movement aimed for "a revolution, but not violent, one of ideas."

Lopez Obrador said he "respects" the movement's independence, and refrained from urging them to join his protests.

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 told foreign reporters.

In a New York Times op-ed, Pena Nieto vowed to fight poverty and "re-examine" the country's drug policies, but also called on the United States to enact immigration reform and do more to curtail demand for drugs.

He said he would create a new 40,000-strong National Gendarmerie to patrol the most violent areas and expand the federal police by 35,000 officers.

Pena Nieto has previously vowed to maintain Calderon's unpopular strategy of using the military to attack the drug cartels and capture crime capos.

The war on the country's powerful cartels in recent years has left a grisly trail of kidnappings, beheadings and mass graves, with the capture of a number of high-ranking kingpins having little effect on the spiraling death toll.

It seemed inconceivable 12 years ago that the PRI would be back in power soon, if ever.

But the drug war—which has killed over 50,000 people during the presidency of Felipe Calderon—handed the PRI a new chance to prove itself.

The economy grew under Calderon, but so did poverty, with 47 percent of 112 million Mexicans considered poor, according to official figures. Josefina Vazquez Mota, the candidate from Calderon's conservative National Action Party (PAN), trailed a distant third on Sunday.

Mexico is Latin America's second biggest economy and is closely tied to the United States and Canada through the North American Free Trade Agreement (NAFTA).

US President Barack Obama called to congratulate Pena Nieto, promising to advance "common goals" with Mexico, including "promoting democracy, economic prosperity and security," according to a White House statement.

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."

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.