The runner-up in Mexico’s presidential election announced Thursday that he will officially challenge the contest's results in court after alleging that the results were manipulated by vote-buying.

Andrés Manuel López Obrador of Mexico’s progressive alliance, which includes the Party of the Democratic Revolution and the Labor Party, finished second to the Institutional Revolutionary Party’s Enrique Peña Nieto after the July 1 election. López Obrador has accused the PRI of buying at least 5 million votes. Peña Nieto was declared the winner by about 3.3 million votes, according to Mexico’s Instituto Federal Electoral’s final vote count.

“Because of this … we can summarize that in free elections, the majority of these citizens would not have voted for Peña Nieto,” López Obrador said during a news conference, according to the transcripts of his speech.

López Obrador and his team officially submitted their court petition to Mexican authorities, along with at least 20 containers that they say contain proof of the PRI’s illegal activity, The Associated Press reported Thursday. The electoral court has until September to render a decision on López Obrador’s challenge and decide whether to officially recognize Peña Nieto as the winner, the AP reported.

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López Obrador, whose narrow loss in the 2006 presidential election led to weeks-long protests in Mexico City, also asked his supporters to remain peaceful throughout the current process.

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