Lawmakers from the party of leftist leader Andres Manuel Lopez Obrador tried to block the ceremony.

NPR: Calderon's Swearing-In Marred by Violence

His opponent, Andres Manuel Lopez Obrador, has dismissed the claim of victory, alleging irregularities in the vote count.

NPR: Calderon Holds Lead in Mexican Vote Recount

The populist candidate, the Democratic Revolutionary Party of Andres Manuel Lopez Obrador, claims that irregularities skewed the race.

NPR: Mexico's Obrador Won't Give Up Presidential Fight