Nieto received congratulations from some leaders for his apparent victory in Sunday's elections.

Official outcome is expected to be released Wednesday and new president will take office on Dec. 1.

Obrador claimed that there were numerous irregularities during the elections.

Mexican Presidential Candidate for Mexico's Institutional Revolutionary Party (PRI) Enrique Pena Nieto (L) and PRI's National President Pedro Joaquin Coldwell (C) attend a press conference in Mexico City, capital of Mexico, on July 2, 2012. Pena Nieto has won 37.93 to 38.55 percent of the vote, according to the quick count results released on Sunday by Mexico's Federal Electoral Institute. (Xinhua/Agustin Horta)

Mexican presidential candidate Enrique Pena Nieto on Monday received congratulatory messages from leaders of some countries for his apparent victory in Sunday's elections.

Nieto declared victory Sunday night after a preliminary count by Mexico's Federal Electoral Institute (IFE), marking the return to power of the Institutional Revolutionary Party (PRI).

The initial results showed that Nieto won 37.8 percent of the vote, about six percentage points more than runner-up Andres Manuel Lopez Obrador of the left-leaning Progressive Movement.

Colombian President Juan Manuel Santos said in an online post that he had spoken with Nieto "to congratulate him for his victory."

"We have a very important agenda with Mexico," he added without going into details.

Mexico's closest European ally, Spain, was one of the first nations to celebrate Nieto's win.

Spanish Prime Minister Mariano Rajoy said in a telegram that Mexico's new leader could count on "all the support of Spain."

"It will always be a priority to work with Mexican authorities to maintain and strengthen ties for the benefit of the people of both counties," Rajoy said.

He also praised "the solid ties of friendship, brotherhood and cooperation" between Spain and Mexico and wished Nieto "good luck in his next tasks."

The White House issued a statement saying that U.S. President Barack Obama called Nieto on Monday to congratulate him.

Obama "reiterated his commitment to working in partnership with Mexico, and looks forward to advancing common goals" such as "promoting democracy, economic prosperity, and security in the region and around the globe."

French President Francois Hollande sent a message via France's embassy in Mexico, saying that the two nations "share the same vision of the great global challenges" and "cooperate closely in international events."

Noting both were elected to office in 2012, Hollande reiterated his "willingness to begin a new phase in the relationship between France and Mexico under the signs of trust and dialogue."

El Salvador's Foreign Ministry issued a statement congratulating the Mexican people "for celebrating the democratic process on Sunday" and congratulating "the virtual winner of the presidential elections, Enrique Pena Nieto."

President Mauricio Funes also expressed "willingness to continue strengthening the solid ties of friendship and cooperation that have united Mexico and El Salvador for years."

On behalf of Nicaraguan President Daniel Ortega, spokeswoman Rosario Murillo said the PRI's return to power is a "historic triumph."

Murillo said Ortega also conveyed his congratulations via a statement sent to both embassies respectively, saying that "for Nicaraguans, it is a cause for joy and satisfaction to recognize the historic triumph of Mexico's Institutional Revolutionary Party in the elections."

The official outcome is expected to be released on Wednesday by the IFE and the new president will take office on Dec. 1.

Other candidates have all concede defeat except Obrador, the runner-up, who said he would wait for the official results. Obrador claimed that there were numerous irregularities during the elections.

Sitting Mexican President Felipe Calderon has also congratulated Nieto for his victory.

The PRI had ruled Mexico for more than seven decades until 2000, when Vicente Fox of the National Action Party (PAN) came to power. The PRI's return to power after 12 years is attributed to worsening crime and poverty under the PAN's rule.