"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.