Mexican Presidential Candidate for Mexico's Democratic Revolution Party (PRD) Andres Manuel Lopez Obrador attends to a press conference in Mexico City, capital of Mexico, on July 2, 2012. Obrador claimed Monday that there were numerous irregularities in Sunday's elections, saying he had a responsibility to defend the electoral process. (Xinhua/Rodrigo Oropeza)

Presidential candidate of Mexico's leftist coalition Andres Manuel Lopez Obrador claimed Monday that there were numerous irregularities in Sunday's elections, saying he had a responsibility to defend the electoral process.

The elections were "plagued by irregularities," Obrador told reporters at a press conference, adding the Federal Electoral Institute (IFE) "has registered more than 3,000 incidents at voting stations, many of them in the State of Mexico."

The IFE said it would announce the official outcome Wednesday.

"Rapid count" results released Sunday evening by the IFE showed that Enrique Pena Nieto, the candidate of the once-ruling Institutional Revolutionary Party (PRI) and a former governor of Mexico State, took the lead with a comfortable margin of six percentage points over runner-up Obrador.

President Felipe Calderon has congratulated the PRI candidate, while two other presidential candidates also conceded defeat in televised addresses.

But Obrador, who has yet to concede, said his team was gathering proof of vote tampering, vote buying and overspending by the PRI.

He also accused the media of lavishing air time on Nieto, an accusation supported by the recent publication of documents that appear to show the purchase of positive television news coverage on behalf of Nieto.

"What happened is a national disgrace," Obrador said of the elections. "I have to act responsibly, and out of respect for those who participated in our movement."

Also Monday, about 20,000 students marched through Mexico City to protest the preliminary election results and were joined by office workers and others, local media reported.

The students shouted slogans against Nieto, the IFE and Televisa, one of Mexico's biggest television networks.