Leftist candidate Andres Manuel López Obrador says he won't concede the Mexican presidency despite an official preliminary count that shows him losing to former ruling party candidate Enrique Peña Nieto, according to latest available reports on Monday morning.

Peña Nieto addresses supporters after early results give him a substantial lead. (Photograph: John Moore/Getty Images ) Nieto, of the conservative Institutional Revolutionary Party (PRI), claimed victory at his campaign headquarters after a preliminary count showed him leading with 38 percent of the vote compared to Lopez Obrador's 31 percent.

"The last word has not been spoken," López Obrador said and vowed to his supporters he would wait until he had "all the information" before stating his formal position on the results.

"Despite claiming the campaign had not been fair and suggesting his own data differed from the official figures released so far," reports The Guardian's Jo Tuckman from Mexico City, "López Obrador's measured tones contrasted sharply with the radicalism that marked his refusal to accept defeat at the last presidential election six years ago. On that occasion his claims of fraud sparked a bitter post-election political crisis that lasted the entire five-month transition period."

The two candidates have squared-off in a contentious election with massive student protests assailing the PRI candidate for corruption and faulting key media outlets for collusion and bias.

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