Obrador said he would do everything in his power to disrupt the administration of President-elect Felipe Calderon.
The left-wing Obrador lost to center-right Calderon by less than 1 percentage point in July's election, claiming widespread fraud cost him the race.
Obrador led a series of protests in the Mexican capital calling for a recount.
Ohio bar shooting arrested, charged with murder
Beautician charged with giving client fatal silicone butt injection