Voters in Mexico have chosen to bring the country's once dominant political party back into power by electing Enrique Pena Nieto as their next president.
With 98 percent of the votes counted, Pena Nieto has 38 percent while former Mexico City Mayor Andres Manuel Lopez Obrador has 31 percent.
Obrador calls the results fraudulent and something no one can accept. His left-wing party will decide whether to formally challenge the results after all votes are counted.
Obrador is accusing Pena Nieto's party, the Institutional Revolutionary Party, or PRI, of buying votes, and says Mexico's news media gave favorable coverage to the PRI, helping to tilt the election in the party's favor.
Obrador says he will challenge the results once they become official, but will not say if he will call for street protests similar to those he led in 2006, when he narrowly lost to outgoing President Felipe Calderon.
U.S. President Barack Obama called Pena Nieto to congratulate him and offer U.S. support in meeting mutual goals.
Pena Nieto told supporters that Mexicans have voted for a change in direction, but he vowed to keep pressure on drug cartels.
"The fight against crime will continue with a new strategy to reduce violence and protect the lives of Mexicans," he said. "Let it be clear, with organized crime there will be no pacts or truce."
Nieto's conservative Institutional Revolutionary Party (PRI) ruled Mexico for 71 years until 2000, when voters elected National Action Party (PAN) candidate Vicente Fox. Outgoing President Felipe Calderon also represents PAN. His administration has been plagued by economic stagnation and rampant drug violence.
Calderon deployed the military to fight the drug cartels shortly after he took office in 2006. More than 50,000 people have been killed.
The PAN candidate in this year's election, Josefina Vazquez Mota, finished third in the voting.
Enrique Pena Nieto, presidential candidate for the Revolutionary Institutional Party (PRI), left, speaks to supporters accompanied by his wife Angelica Rivera at the party's headquarters in Mexico City, early Monday, July 2, 2012.
Enrique Pena Nieto greets supporters after exit polls showed him in first place, in Mexico City, July 1, 2012.
Supporters of Enrique Pena Nieto celebrate at party headquarters as exit polls begin to come in, Mexico City, July 1, 2012.
Supporters of Enrique Pena Nieto celebrate at the PRI party headquarters as results begin to come in on the general elections in Mexico City, July 1, 2012.
Supporters of Enrique Pena Nieto gather at their party's headquarters in Mexico City, Mexico, July 1, 2012.
residential candidate Andres Manuel Lopez Obrador of the Democratic Revolution Party (PRD) speaks in Mexico City, July 1, 2012.
Some information for this report was provided by AP, AFP and Reuters.