On Wednesday, officials undertook the final count based on a review of polling station reports. With 94% of the polling stations tallied, Lopez Obrador led Calderon by 0.73 percentage point, or about 288,000 votes.

Representatives of Lopez Obrador's Democratic Revolution Party said the reversal was caused by the discovery of thousands of votes for their candidate that had gone uncounted in the preliminary tally.

At counting centers across Mexico, they demanded that individual ballot boxes be opened for recount. Although Calderon representatives objected, many boxes were opened and the votes tabulated.

Leaders of Calderon's National Action Party insisted he would regain his lead when the votes were counted in northern states that are PAN strongholds.

Votes from northwestern Mexican states such as Baja California and Sonora were being reported later because they are on Pacific time, two hours behind most of the rest of Mexico, PAN officials said. Those states favored Calderon in preliminary tallies.

Across Mexico, the recount was an exercise in community democracy, with ordinary citizens joining party activists and election officials to witness the tallying of the reports at 300 regional offices of Mexico's Federal Election Institute, three days after an estimated 42 million Mexicans cast their votes.

Coming after a campaign that was one of the nastiest the country has seen, the drama of the vote count served as an emotional and divisive experience for many Mexicans, much as the 2000 Florida recount saga was for many Americans.

When the last results of the preliminary count were released Tuesday evening, Calderon was leading by 0.6%, or 257,000 votes. Early Wednesday, there was no hint in the media of a possible reversal, and Calderon was being treated as the likely winner.

But as the count moved ahead, Lopez Obrador quickly jumped into the lead, and by noon, he was ahead by about 2.5%. Later in the day, Calderon inched closer.

Cesar Nava, a top PAN official, predicted that when all the votes were counted, Calderon would win by 300,000.

In an early-afternoon radio interview, Nava accused PRD activists of trying to intimidate polling officials, saying they were fabricating excuses to open the ballot boxes for recounts.

Lopez Obrador staffers said that many boxes held uncounted votes for their candidate.

"The official count is finding votes [for Lopez Obrador] that the preliminary count missed," said Horacio Duarte, the PRD's representative before the Federal Election Institute, the independent government body in charge of organizing the vote.

El Universal newspaper reported on its website that in one electoral district in Lopez Obrador's home state of Tabasco, Wednesday's count had turned up an additional 20,405 votes for the leftist candidate.

On Wednesday morning, as the official tally of polling place reports was beginning, PRD officials said they would not recognize it because they had discovered widespread fraud. They held to that position even when the count showed their candidate leading.

They insisted that all 42 million votes be recounted, but election officials said the law provided for the reopening of ballot boxes only when local officials noted irregularities in the paperwork assigned to each box.

Leonel Cota, president of the PRD, said the vote risked becoming "an election of state," a phrase referring to the farcical votes of Mexico's recent past that always reelected government candidates.

Each of the five parties with presidential candidates had representatives at the counting stations, and Lopez Obrador called on his supporters to act as unofficial observers. With crowds of onlookers cheering and booing, PRD representatives often insisted that ballot boxes be reopened for recounts.

The party mobilized scores of demonstrators outside some district offices and sent others inside to film the vote count with hand-held cameras.

About 200 PRD activists shouting "No to fraud!" marched from Mexico City's independence monument to the presidential residence, where they were stopped by an equal number of riot police and peacefully dispersed.

At the District 10 office in the San Miguel Chapultepec neighborhood of Mexico City, sealed white boxes that had been in the custody of the Mexican army since Sunday's vote were cut open beginning 8 a.m. Wednesday. The tally sheets were read aloud before representatives of the five parties in the race and entered into the electoral system's computerized database.

The seven-member district electoral board in the overwhelmingly PAN neighborhood voted down all but one of the PRD's six appeals to recount ballots in individual boxes.

Federico Martinez, the PRD representative at the District 10 counting table, objected. He insisted that one ballot box the electoral board refused to recount contained two ballots more than reported on the tally sheet.

Word of the conflict filtered outside the building, riling the small crowd of protesters.

"Multiply those two votes by thousands of polling places and they add up," said Lidia Andrade Rodriguez, 43. "We're going to fight for those two votes."

The seven election officials conducted the count around a large table, in the presence of party representatives and a score of independent observers. The scene bore a faint resemblance to the examination of Florida's infamous hanging chads.

Safeguards built into the system for transparency were evident. The table was piled high with copies of the tally sheets kept by each party's poll watchers as a check on the official sheets pulled from the ballot boxes. The laptop computer on which the tallies were entered was projected onto a wall for everyone to see.

Across town at the Tlalpan district counting office, the PRD challenges were more effective. The electoral board there accepted its first seven requests for recounts, resulting in a gain of 310 votes for Lopez Obrador.

Each recount was a mini-drama, conducted by Marineyla Huerta, a formidable economist serving as the electoral board chairperson, and watched intensely by dozens of party representatives and observers in the crowded room.

Wielding a green box cutter, Huerta opened the seventh box and separated the ballots by candidate preference on the table before her. Then she proceeded to count each pile, slowly and out loud.

As she leafed through the pile for Lopez Obrador, she accidentally counted two ballots as one.

"Wait! You're miscounting!" someone shouted. A PRD activist with a mini-cam filmed the scene from up close.

Huerta checked and conceded her error.

"I'll start over," she said.

The recount confirmed that the box had contained 11 ballots more than were listed on the tally sheet. Ten of those votes were for Lopez Obrador.

"How interesting, no?" Huerta said.

As the review continued, the chants of about 30 PRD supporters from across the street echoed through the counting room.

"Obrador! Obrador!" they shouted.

But many Mexicans said they were simply exhausted by the controversy.

Stuck in traffic in central Mexico City caused by the PRD rally, Alejandro Ortega Perez, 54, decided to abandon the taxi taking him to the airport and walk around the protesters.

"I hope this is over soon and that they tell us once and for all who won, and that everyone accepts it," Ortega Perez said. "They need to let us citizens live in peace, whoever wins."

Times staff writers Cecilia Sanchez and Carlos Martinez contributed to this report.