Election workers across Mexico began a marathon review of vote tallies Wednesday to determine whether conservative candidate Felipe Calderon really won the country's tight presidential race.

Calderon's leftist challenger still insisted he was the victor and denounced what he called widespread irregularities.

A preliminary count showed Calderon, of President Vicente Fox's National Action Party, ahead by 1 percentage point.

Charismatic leftist Andres Manuel Lopez Obrador is demanding a recount of every ballot, saying Wednesday: "The political stability of the country hangs in the balance."

Lopez Obrador could mobilize millions if he doesn't get his way. In a news conference Wednesday, Calderon raised the possibility that he would do the same.

"We could also call for protests, but the vote can't be replaced by demonstrations," he said.

He criticized Lopez Obrador's allegations that the vote was tainted, saying: "They are trying to undermine an election without having the results to back it up."

Federal Electoral Institute President Luis Carlos Ugalde said late Tuesday that 2.6 million votes were not included in the preliminary count because of "inconsistencies," such as poor handwriting or extraneous marks on the tally sheets attached to each ballot box. Lopez Obrador initially had said those ballots were "missing."

If a recount of the votes inside prove the numbers on these tally sheets are valid, Calderon would still lead, but by just 0.64 percent -- about 250,000 of the 41 million votes cast -- Ugalde said.

Electoral officials across Mexico began early Wednesday to review all the tally sheets, a process that by law must continue round the clock until the final, official result is determined. It was unclear when that might be.

The final count will be turned over to Mexico's electoral courts, where political parties can dispute the results. The electoral court certifies the winner on September 6.

Ugalde said officials will open ballot boxes to conduct individual counts only where there are tally sheet problems.

Observer: 'Nightmare scenario'

In an initial meeting between electoral officials and party representatives, Horacio Duarte of Lopez Obrador's Democratic Revolution Party demanded that officials recount all the votes "for the health of the republic."

"There is one thing missing in this process: certainty," Duarte said.

National Action's German Martinez responded: "They want to throw out the election because it didn't favor them. Pressure and blackmail should not prevail."

The review that began Wednesday is a crucial step in proving the elections were clean to a nation that emerged six years ago from 71 years of one-party rule replete with election fraud. Failure to convince the public and candidates it has been a fair vote could spark widespread civil unrest.

"Such a close race is a nightmare scenario," said Ted Lewis, an election observer for the San Francisco, California-based Global Exchange. "If the ruling party wins by a hair, a lot of people will jump to the conclusion that something is amiss."

Lopez Obrador aide Claudia Sheinbaum said Wednesday that the party found "very grave inconsistencies" in at least 50,000 polling places, including 18,646 in which votes cast outnumbered ballots distributed by officials. There was no immediate response from election officials.

"Make the review thorough so all will be satisfied, so we can end this process cleanly," Lopez Obrador said.

About 35 Lopez Obrador supporters set up camp Wednesday outside the gates of the Federal Electoral Institute, known as the IFE, draping banners that accused electoral officials of being traitors.

"We are just trying to defend our democratic institutions," said Alejandro Robles, a 22-year old student. "We carried out our duty: to vote. Now we see the actions of the electoral officials as very suspicious."

Electoral institute has eased fears of fraud

Mexico became accustomed to widespread accusations of electoral shenanigans during seven decades of rule by the Institutional Revolutionary Party, or PRI.

Fears of such fraud have sharply diminished since the Federal Electoral Institute was founded in 1990s. The autonomous institute oversaw Fox's peaceful defeat of the PRI in 2000 and has been held up as an example of a world-class electoral body that has advised emerging democracies, including Iraq and Haiti.

Interior Secretary Carlos Abascal told reporters Tuesday that the Fox administration "does not endorse the victory of anyone, at all."

But political analyst Jose Antonio Crespo said there is still concern that some Federal Electoral Institute officials are too close to Calderon and his ruling party. Crespo said the institute needs to be scrupulous in its review to prove Mexico's democratic credentials.

"I am worried," he said. "All the players must tread very carefully or this situation could escalate out of control."

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