Mexico began a tortuous review of the more than 41 million votes cast in Sunday's disputed presidential election today, as the gap between the two leading candidates, one a conservative, the other a leftist, appeared to narrow.

Preliminary results from Mexico's Federal Electoral Institute (IFE) gave Felipe Calderon, a former energy secretary and the candidate of the ruling National Action Party (PAN), a 1 per cent or 402,000-vote lead over Andres Manuel Lopez Obrador, a former mayor of Mexico who has promised the redress the gap between the rich and the poor.

But last night, officials from the IFE revealed that if 2.6 million votes showing "inconsistencies" such as poor handwriting were included in the count, the gap would close to just 257,000 votes, or 0.6 per cent. As many as 2 million further votes are reported not to have been counted at all.

Both candidates declared victory in the election on Sunday night and neither has backed down.

As the razor-thin difference between the totals has become even sharper, Señor Lopez Obrador has increased the pressure on Mexico's electoral authorities by alleging that supporters of Señor Calderon's party have counted votes twice in some districts and ignored ballots in others.

"The stability of the country is at stake," he said this morning at a news conference as officials started to open ballot boxes in Mexico's 138,000 polling stations, looking for suspicious totals. "Make the review thorough so all will be satisfied, so we can end this process cleanly."

One of Señor Lopez Obrador's aides, Claudia Sheinbaum, went on to claim that his Party of the Democratic Revolution (PRD) had found "very grave inconsistencies" in 50,000 polling booths, including 18,646 in which there were more votes cast than registered voters.

The election review is expected to deliver a definitive result by Sunday, but legal challenges could delay any resolution for weeks or months. Bitterly contested parliamentary elections have already prevented either party from winning a clear majority in Mexico's legislature.

The unease surrounding the count has been exacerbated by fears that Señor Lopez Obrador's fervent supporters, many of whom are poor, urban and eager to see the wholesale reform of Mexico's public services, could take to the streets. The peso has wobbled back and forth against the dollar as the likelihood of a Calderon government, thought to be more favourable to business, has fluctuated.

Señor Lopez Obrador has repeatedly referred to Mexico's indifferent record of democratic elections to justify his suspicions of the present contest. The PRD was famously cheated of the presidency in 1988, when its leader, Cuauhtémoc Cárdenas, who was thought to have won the popular vote, was denied by a series of computer crashes.

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

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