A seven-judge panel - known as the TEPJF - is weighing 364 challenges to an election where an official count gave conservative former Energy Secretary Felipe Calderón a slight advantage over López Obrador, who stepped down as Mexico City´s mayor to run for president.

The closed-door session was the first hearing since the disputed vote and featured arguments from three attorneys representing López Obrador´s Democratic Revolution Party (PRD) and the coalition under whose banner the former mayor ran.

An official count the week following the election gave Calderón an advantage of less than 0.6 percent, about 240,000 votes out of 41 million cast. A president-elect can´t be named until the Federal Electoral Tribunal rules, and it must either declare a winner or annul the election by Sept. 6.

López Obrador, a fiery populist who can mobilize millions, says widespread fraud and a dirty campaign are responsible for Cal- derón´s narrow lead. He has called for a vote-by-vote recount he says will prove he was the winner.

At a news conference, the lawyers said they outlined for the judges mathematical errors, falsifications or other problems at 72,000 of 130,000 polling places.

President Vicente Fox - a member of Calderón´s National Action Party (PAN) - "inept," and accused his administration of using its power and money to influence the vote.

After weeks of protests and allegations that Mexico is slipping back to its fraudulent electoral past, many have begun to question the Federal Electoral Institute (IFE), the independent body responsible for organizing the election. López Obrador and his supporters have called for the full recount they demand to be conducted by officials not associated with the institute.