MEXICO CITY - Felipe Calderon was acting presidential Wednesday, telling The Associated Press that he would happily give his leftist rival a Cabinet post and practically skipping down the hall as supporters called out "presidente!"

But Andres Manuel Lopez Obrador wasn't ready to settle for anything less than the presidency. He insisted on a ballot-by-ballot recount he said would expose "serious evidence of fraud" in a preliminary tally that gave Calderon the slimmest of victories.

Electoral officials said they had no authority to order a recount of all 41 million ballots cast Sunday, and proceeded with their official count of tally sheets to determine who is declared president.

That marathon count began in 300 district headquarters across the country Wednesday morning. With 56 percent of the sheets counted later Wednesday, Lopez Obrador had 37 percent compared with 35 percent for Calderon.

In an exclusive interview with AP at the National Action Party's headquarters, Calderon said he would be willing to include Lopez Obrador in his Cabinet as he builds a coalition government. But he didn't think his opponent would accept, saying they haven't even spoken since the election.

The preliminary count completed earlier in the week had Calderon winning by 1 percentage point. Leonel Cota, president of Lopez Obrador's Democratic Revolution Party, accused election officials of deliberately mishandling that count to confirm a win for Calderon, the ruling-party candidate. He said Lopez Obrador won Sunday's vote.

"We are not going to recognize an election that showed serious evidence of fraud, that was dirty from the start, manipulated from the start," he said.

When polls closed, citizens staffing the 130,488 polling places opened the ballot boxes and counted the votes, then sealed them into packages with their tallies attached and reported unofficial totals to the Federal Electoral Institute, or IFE. The institute then posted preliminary results on its Web site from about 41 million ballots cast.

The sealed packages were delivered to district headquarters, where elections workers used the tallies Wednesday to add up the formal, legal vote totals.

Workers were not reviewing individual ballots except when the packages appeared tampered with or their tallies were missing, illegible or inconsistent - including at least 2.6 million ballots likely to shrink Calderon's lead to 0.64 percent if included, election officials said Tuesday.

At one electoral office in Mexico City, officials opened a ballot box because the vote tally was missing. The votes were then re-counted out loud while 10 party representatives stood by with tape recorders and video cameras.

"I'm exhausted. I'm still tired from election day," said counter Rocio Sanchez, 41, an IFE employee. "But this is something we have to do by law."

Cota said Democratic Revolution would not recognize the results without a ballot-by-ballot recount. But IFE President Luis Carlos Ugalde said that was not possible.

"Mexican law is very clear on when a ballot box can be opened: only when there are problems with the vote tallies, when the tally sheet has obviously been changed, or when the box has been tampered with," Ugalde said.

Once the count is complete, the seven-judge Federal Electoral Tribunal hears any complaints and can overturn elections. By law, it must certify a winner by Sept. 6, and its decision is final.

Cota said the party might take its case to international tribunals.

Ugalde scolded both candidates for prematurely declaring victory, saying: "No political party can declare or affirm, at this time, that its candidate has received the largest number of votes."

Lopez Obrador called again Wednesday for his supporters to remain calm, but he could mobilize millions - as he has in past legal disputes - and he hinted Wednesday that he might.

"The political stability of the country hangs in the balance," he said.

In the AP interview, Calderon said demonstrations would be irresponsible. "Elections are not won on the street," he said. "They are won in the voting places."

The review that began Wednesday is a crucial step in proving the elections were clean to a nation that emerged only 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 unrest.

"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."

Most international observers said the election was fair and properly carried out by Mexico's world-renowned system, held up as a model to emerging democracies in Iraq and Haiti.

There have been fears that the battle over the presidency could turn violent. There were scattered protests Wednesday in favor of Lopez Obrador, all of them peaceful.

About 35 people set up camp Wednesday outside IFE's gates, draping banners that accused electoral officials of being traitors, and about 300 protesters marched down Mexico City's broad Reforma Avenue carrying a banner reading: "Respect Andres Manuel Lopez Obrador's victory!"

"We're not going to let them get away with this," said 62-year-old Enrique Flores, a retired Mexico City school teacher.

- Associated Press reporters Ioan Grillo, Regina Reyes-Heroles and Will Weissert contributed to this report.

Past incidents of fraud in Mexico presidential elections

1988: Early returns show leftist Cuauhtemoc Cardenas leading, but a computerized election system mysteriously fails, and when it is reconnected hours later, Institutional Revolutionary Party, or PRI, candidate Carlos Salinas wins by a narrow margin.

1958: PRI candidate Adolfo Lopez Mateos defeats National Action Party, or PAN, candidate Luis H. Alvarez with 90.56 percent of the vote. Alvarez was not allowed any radio time and was jailed during the campaign on the charge of "being an opposition candidate."

1940: PRI candidate Manuel Avila Camacho wins with 93.9 percent of the vote over independent Gen. Juan Andrew Almazan in an election marred by violent clashes and credible accusations of vote fraud. A number of Almazan's followers were persecuted and killed.

1910: Riots break out after elections widely considered rigged give another term to Porfirio Diaz, who rules Mexico for 30 years with an iron fist. Officials say opponent Francisco I. Madero only received a few hundred votes in the entire country. Madero calls the Mexican people to take up arms and fight against the government, launching the Mexican Revolution.

Allegations of voting irregularities made by Mexico's leftist presidential candidate Andres Manuel Lopez Obrador or his party:

- At least 51,906 polling places had "grave inconsistencies." Of those, 781 had more votes cast than there were registered voters and 18,646 had more votes than they had ballots. It was unclear what the rest of the irregularities were.

- Some votes appear to have been counted twice, while others were not tallied at all.

- There were differences between what some poll workers tallied and what official computerized systems recorded.

- A high number of votes - more than 3 million - had been believed to be "missing." (Electoral officials later said - and Democratic Revolution agreed - that these votes had been set aside because of possible mistakes on the tally sheets and are now being included in the official count.)

- Many polling-station tallies showed significantly fewer presidential votes than the votes cast in local races.