The first of Mexico's presidential debates has opened with fire, with front-runner Andres Manuel Lopez Obrador defending the idea of an amnesty to fight drug violence and reviving his famous campaign slogan from 2006, "For the good of all, the poor come first."

The leftist candidate defended his proposals to "speak with everybody" to solve Mexico's crime problem. He said he would even invite the pope to discuss how to carry out such a dialogue.

The other candidates harshly criticized the amnesty proposal. Third-place candidate Jose Antonio Meade says the proposal puts Lopez Obrador "on the side of the criminals."

Lopez Obrador sought in his failed 2012 presidential bid move to the middle and hadn't much mentioned the "poor come first" slogan of this unsuccessful 2006 campaign until Sunday.