Poll Loser's Supporters March on Mexico City

Supporters of the charismatic leftist Andrés Manuel López Obrador are marching across Mexico towards the capital protesting against the "fraud" they believe cheated him out of victory in the recent presidential election.

Mr López Obrador came second in the official count of the July 2 poll, just 0.6%, behind the conservative governing party candidate Felipe Calderón.

Organisers said 40,000 people set off on Wednesday from towns and cities around the country on the "March for Democracy", aiming to converge on Mexico City at the end of the month.

"I hope that it will be an extraordinary, gigantic march that reflects the anger over the fraud," said Jesús Ortega, a leading aide to Mr López Obrador.

Each branch of the protest will also send advance delegations to a demonstration on Sunday in Mexico City which many expect to attract more than 500,000 people. The nationwide mobilisation takes place as the country's electoral tribunal considers Mr López Obrador's legal challenge to the result filed last Sunday.

Mr López Obrador's supporters say they want the tribunal to order a vote-by-vote recount, although the case also appears to argue in favour of declaring the election void.

The López Obrador camp argues the count was subject to irregularities reminiscent of the rigged polls that helped sustain one party in power in Mexico from 1929 to 2000. Almost every day this week Mr López Obrador has revealed fresh evidence which he says proves fraud is as widespread as ever in the country.

Not all the evidence has worked in his favour. A video of a man apparently stuffing a ballot box at one polling station turned out to be an innocuous operation to move votes from one box to another that was approved by a representative from Mr López Obrador's own party.

Mr Lopez Obrador's camp also claims the electoral authorities manipulated the presentation of partial results from the count in an attempt to prepare public opinion for a staged result.

The tribunal has until September 6 to rule on the challenge, declare the election valid and name a president-elect.

Mr Calderón is trying to give the impression that the dispute is a formality. Congratulatory letters and phone calls from foreign leaders - including Tony Blair - have helped, along with the local mass media's tendency to question Mr López Obrador's motives.

But Mr Calderón was criticised this week for announcing a national victory tour and for naming a "transition team" to prepare for the December 1 handover of power.

There are also a growing number of non-partisan voices calling for a vote-by-vote recount to help restore legitimacy to the electoral system.