MEXICO CITY (CNN) -- The political party that ruled Mexico for more than 70 years appears poised to return to power after election authorities projected Enrique Peña Nieto as the winner of the nation's presidential vote.
A quick count based on samples from polling stations throughout the country gave Peña Nieto the lead, with between 37.93% and 38.55% of votes, the Federal Election Institute said late Sunday night.
The projected victory for Peña Nieto marks a triumphant return to power for the Institutional Revolutionary Party (PRI), which lost its grip on Mexico's presidency to the conservative National Action Party in 2000.
"I take with great emotion and a great sense of commitment and full responsibility the mandate Mexicans have granted me today," Peña Nieto told supporters, standing at a podium with a sign that said "Mexico won."
But leftist candidate Andres Manuel Lopez Obrador, Peña Nieto's closest competitor, said Sunday night that he wasn't ready to concede.
"The last word has yet to be said," the former Mexico City mayor told supporters in the capital late Sunday. He trailed by 6 percentage points in the Sunday night quick count, which projected he garnered between 30.90% and 31.86% of the vote.
An official individual vote tally begins Wednesday.
Sunday's election projections raise two key questions rooted in Mexico's complicated political past: Has the PRI, a political party that critics accuse of being authoritarian and corrupt, changed its approach? And will supporters of Lopez Obrador protest election results as they have in the past?
Peña Nieto said Sunday night that he is looking forward, not back.
"We are a new generation. There is no return to the past. My government will have its vision based in the future," he told reporters.
On the local level, there may not be many differences between today's PRI and the political party that dominated Mexico for decades, said Andrew Selee, director of the Mexico Institute at the Woodrow Wilson International Center for Scholars.
"What's changed on the national stage is that Mexican citizens have different expectations for their federal government that are going to force the PRI to govern in a different way than it did 20 years ago," he said. "Then, the PRI was really a party that included all of Mexico, that had a broad patronage network and tolerated little dissent outside of the party. And the PRI today is going to have to deal with opposition parties that have tasted power, an active citizenry that expects to be involved in major policies decisions, and a very vigilant press that will report on everything that happens."
Weeks before Sunday's vote, criticisms of Peña Nieto and concerns about the PRI's possible return to power fueled a student movement that has staged demonstrations throughout the country.
In the 2006 presidential vote, election authorities said Lopez Obrador narrowly lost to Felipe Calderon. Lopez Obrador claimed election fraud and never conceded, referring to himself as "the legitimate president of Mexico."
His supporters protested nationwide. In Mexico City, they staged sit-ins and blockades.
Last Wednesday, Lopez Obrador told throngs of supporters in Mexico's capital that he was confident that he would "win the presidency again."
But while Lopez Obrador's supporters slam Peña Nieto, the charismatic 45-year-old former governor has galvanized fervent support among residents of his home state and party loyalists nationwide.
The Consulta Mitofsky, GEA/ISA and Parametria firms said their exit poll results projected a win for Peña Nieto, with more than 40% of voters saying they cast ballots for the PRI candidate.
Peña Nieto's campaign platform included plans to stop the rise in food prices, promote energy reform, give social security to all Mexicans and reduce violence nationwide.
"He is obviously prepared. There was obviously a dirty war against him," said Martha Rojas Ramos, 58, as she prepared to cast a ballot for Peña Nieto at a Mexico City elementary school on Sunday.
Critics lamenting the possible return of the PRI to power aren't thinking straight, she said.
"That's all in the past. What's important is that he is young and has all the ability to represent us," she said.
Alejandro Garcia, a 33-year-old accountant, said he supports Peña Nieto's security strategy, which aims to decrease violence in Mexico.
Calderon, Mexico's current president, made combating cartels a top priority when he took office in December 2006. Since then, more than 47,500 people have died in drug-related violence nationwide, according to government statistics.
Garcia said the surge in violence has negatively impacted daily life throughout the country.
"People don't go out as much in the streets. People go inside their homes earlier in the day. ... Now we are seeing things that we didn't see before. Maybe they were going on, but they weren't as open as they are now," he said. "I think (Peña Nieto) is the one to stabilize the country."
In addition to Peña Nieto and Lopez Obrador, two other candidates vied for the presidency Sunday in what officials called "the largest and most complex election day" in the country's history.
Ruling party candidate Josefina Vazquez Mota was trailing in exit polls and the quick count projection, which said she received between 25.10% and 26.03% of votes.
As preliminary results trickled in Sunday night, the National Action Party candidate acknowledged that the trend did not appear to be in her favor.
Gabriel Quadri of the New Alliance, who lagged far behind in polls before and after the election, praised Mexico's election authorities Sunday night.
"We have very solid, democratic institutions," he said.
But some voters said they were afraid of fraud.
From a command center in Mexico's capital, student activists tracked election irregularities reported in local media, and encouraged others to document activities at their neighborhood polling stations.
The monitoring effort was spearheaded by youth who have led a series of social media campaigns and street protests preceding Sunday's vote.
More than 2,100 federal, state and local offices were decided by Sunday's vote, according to Mexico's Federal Election Institute.
Voters elected governors in the states of Chiapas, Guanajuato, Jalisco, Morelos, Tabasco and Yucatan. In Mexico City, the nation's capital, residents elected a new mayor.
Authorities estimate that more than 49 million Mexicans voted, 62% of the registered voters.
"Today's election is the election in which the most votes have been cast in Mexico's history," said Leonardo Valdes, president of the Federal Election Institute.