Andres Manuel Lopez Obrador at the Zocalo plaza in Mexico City

The man who lost Mexico's presidential election in July has been sworn in as the head of a parallel government he has formed.

Andres Manuel Lopez Obrador Monday declared himself Mexico's "legitimate president" as he donned a replica of the red, white and green presidential sash for the ceremony, which had no legal weight. Lopez Obrador told thousands of supporters he would work for the poor.

Lopez Obrador was defeated by conservative Felipe Calderon, who will take office on December 1. Mr. Calderon replaces fellow conservative Vicente Fox, who is constitutionally barred from seeking a second six-year term.

Lopez Obrador says his shadow government will try to prevent the ruling conservatives from doing whatever they wish. His supporters have threatened to disrupt Mr. Calderon's inauguration.

Lopez Obrador's event coincided with Mexico's celebration of its 1910 revolution.

Lopez Obrador contested his loss to Mr. Calderon, claiming vote fraud and charging that President Fox used the power of his office to support the incoming president.

Mexico's electoral tribunal rejected Lopez Obrador's claims.

Some information for this report was provided by AFP, AP and Reuters.