More than 100,000 defiant supporters of leftist presidential candidate Andres Manuel Lopez Obrador massed Saturday in a bid to overturn his narrow election defeat with protests that threatened to widen Mexico's regional and class divisions.

Lopez Obrador told the huge crowd he would present allegations of fraud to the nation's electoral court Monday and request that every one of more than 41 million votes be recounted, to expose what he called wrongdoing that cost him the election.

"We are going to ask that they clean up the elections. We are going to ask that they count all the votes — vote-by-vote, poll-by-poll," Lopez Obrador said to wide applause.

Lopez Obrador says he will challenge the result in electoral tribunals and in the Supreme Court. He claims hundreds of thousands of votes for him remain uncounted, miscounted or voided.

These days, Lopez Obrador must walk a tightrope. If he appears too radical, he risks hurting his party and its chances in the next presidential elections in 2012. If he appears too moderate, he risks disappointing his core supporters.

"His political stock would increase greatly for 2012" if he finds a way to concede defeat gracefully, political analyst Oscar Aguilar said.

He has in the past headed protests that turned disruptive or violent.

In 1996, he led farmers and fishermen in sometimes-violent takeovers of state-owned oil wells to demand compensation for damages from an oil spill.

Last year, as Mexico City mayor, he led huge street protests that forced Fox to fire his attorney general and drop a legal case that would have kept Lopez Obrador out of the presidential race.