Mexico's leftist candidate Andres Manuel Lopez Obrador said Monday that his rival Enrique Pena Nieto had bought 5 million votes that helped him claim victory in the presidential elections.

According to the final tally released Sunday, Nieto of the Institutional Revolutionary Party (PRI) won with 38.21 percent of the vote against 31.59 percent for Obrador of the center-left Progressive Movement.

"We have supportive evidence, and we will prove that they bought about 5 million votes," Obrador told a press conference.

Obrador complained that the Federal Electoral Institute did not punish the alleged "vote-buying" behaviors of the PRI, who had ruled the country for seven decades in the last century and would return to power after 12 years in opposition since 2000.

Mexico President Felipe Calderon said vote-buying was "simply unacceptable" and "a defect in our democratic quality that must be corrected immediately," but he questioned whether these allegations would be sufficient enough to overturn the election result.

Obrador pledged to purse a legal process in a peaceful manner.

"We will continue the legal process. Our movement has been, is and will remain peaceful. We will not compromise in defense of democracy," Obrador said.

Obrador lost the 2006 presidential elections to Calderon, and his claim for fraud had caused chaos in central Mexico City for months.