More than 100,000 protesters filled a vast Mexico City square yesterday to back leftwing presidential candidate Andres Manuel Lopez Obrador, who says elections were rigged against him.
To cheers, whistles and shouts of "Fraud, fraud", Mr Lopez Obrador told the crowd the official razor-thin election victory of the conservative ruling party's Felipe Calderon last Sunday was bogus.
He said he was the legitimate winner and called for more protest marches to pile on the pressure as his legal team challenges the results before Mexico's highest election court.
"We are going to defend democracy," Mr Lopez Obrador shouted. "We are going to ask that they recount all the votes, vote by vote, polling station by polling station. All the ballot boxes must be opened because there is solid evidence that they took out votes to favor the candidate of the right."
The leftist campaigned on a promise to pull millions of people out of poverty with ambitious welfare programs and infrastructure projects, but his policies and crowd-pleasing rhetoric alarm business leaders and investors.
About 130,000 people came out in support of the popular former mayor of Mexico City, filling Zocalo square with the yellow flags of his party and shouting "Obrador, Obrador."
Some held posters reading "No to the damn fraud", accused the ruling party of vote-rigging and some warned of unrest. "If there's no solution, there'll be a revolution," they chanted.
Mr Lopez Obrador will challenge the result in the electoral court, but Mr Calderon is already looking presidential after a recount of vote tally sheets showed he won by less than 1 percentage point.
US President George W. Bush and Spanish Prime Minister Jose Luis Rodriguez Zapatero annoyed Mr Lopez Obrador when they called his rival on Friday to congratulate him on his win.
Mr Lopez Obrador said the party of Mr Calderon and President Vicente Fox had "learned fast" the dirty tricks used by the Institutional Revolutionary Party that ruled for 71 years until Mr Fox toppled it in 2000 in Mexico's cleanest election to date.
He said count figures flashed on television had been rigged and were not consistent with the real vote count. "The electronic counting system was manipulated. Mathematically, we have the proof of how the manipulation was done," he told foreign reporters.