Around 600 international visitors were in Mexico to oversee the contest. The majority initially said the elections were clean and transparent, but with allegations of irregularities persisting, some groups are now calling for clarification from electoral authorities.

Ted Lewis, who headed a delegation of observers from Global Exchange, a San Francisco-based non-profit organization, said his team witnessed vote buying first-hand in the states of Oaxaca and San Luis Potosí.

"In a normal electoral contest that wasn´t so razor-close, these kind of things really wouldn´t weigh in to the outcome of the presidential election," Lewis said.

Sunday´s contest was not a normal election, however. According to preliminary counts - challenged by the Democratic Revolution Party (PRD) - Felipe Calderón, of the National Action Party (PAN), finished with a lead of less than 1 percentage point over PRD candidate Andrés Manuel López Obrador.

Late Wednesday, electoral workers at the 300 elections districts were busy compiling the election´s official tally that will be used to determine the winner.

But more alarming than the vote-buying, Lewis said, were discrepancies in the vote count found by statistical specialists working for Global Exchange and its Mexican partner Alianza Cívica (Civic Alliance), which also observed the polling process.

"We´re not at the point where we´re ready to say electoral fraud but we´re really quite disturbed," Lewis said. "We didn´t expect to see this level of irregularity."

He said the groups´ experts had found that the number of votes for Congress in various instances exceeded the number of votes for the presidency in states where López Obrador reportedly won, while the opposite was true in states where Calderón was the victor.

He said the Federal Electoral Institute (IFE), the body charged with overseeing the contest, had denied any irregularities exist in a meeting with Alianza Cívica.

"At this point, we´re frankly quite alarmed by what we´re seeing and the fact that this really could be a kick in the gut for the democratic process in Mexico," Lewis said. He added that while his organization has supported the IFE in the past, currently "the confidence of the electorate is dropping by the hour."

Other observers, such as Robert Pastor, who heads centers on North America and democracy at American University in Washington, defended the IFE and said the candidates - both of whom declared themselves victors shortly after the election was declared too close to call - need to practice more restraint.

"The IFE is simply one of the best election organizations in the world, and Mexico will need that in the days ahead," said Pastor, who has observed four Mexican elections since the ´80s. "But at the same time they still have a very primitive democratic culture. So there will be a struggle between the new rules and the old habits."

The old habits, he said, were to pressure the state through demonstrations or to strike behind-the-scenes political bargains, while the new rules are "to settle based on the courts and who wins the most votes."

Pastor said both parties were behaving irresponsibly, adding that fraud was unlikely and inconsistencies were more likely attributable to human error. He said that recounts were necessary only at polling stations where numerical discrepancies were reported.

At the press conference earlier, PRD President Leonel Cota called for a recount of 100 percent of the ballots, and refused to directly answer questions on whether the party would accept the result if only the contested polling stations were recounted.

Pastor acknowledged the political tensions were threatening stability and called for patience.

"I think there is real suspicion among the PRD and I think it will require real leadership on the part of López Obrador and his colleagues to keep people calm and help them to realize that there is a legal process underway and that they´re not going to be victimized," Pastor said.