MEXICO CITY - Felipe Calderon, a Harvard-educated conservative, was declared the winner of Mexico's presidential race Thursday, but the celebration was spoiled by his leftist rival's vow to contest the result in court and in the streets.
Calderon, who at 43 would be Mexico's youngest president, won the official count by half a percentage point, or 243,934 votes out of nearly 41 million, over former Mexico City Mayor Andres Manuel Lopez Obrador in the most bitterly battled election in Mexican history.
At a victory party, Calderon called for the country to put aside its differences. But before the results were final, Lopez Obrador announced he was appealing to the federal electoral court to prove the election was rigged in favor of the candidate of outgoing President Vicente Fox's ruling party.
For the first time since Sunday's deadlocked election, Lopez Obrador called out his passionate followers for a mass rally Saturday night in Mexico City's central square. He said it would be a peaceful ''informative assembly,'' but it also will be a show of force to pressure electoral authorities.
''There's nothing to celebrate,'' a subdued Lopez Obrador said Thursday as the last votes were being tallied in a countrywide official count. ''They know what they did.''
The court has until Aug. 31 to rule on Lopez Obrador's complaint and until Sept. 6 to officially name the next president. Unlike the Federal Electoral Institute, or IFE, which conducted the vote, the court has authority to open every ballot box and count every vote, one of Lopez Obrador's primary demands.
The seven-judge court has final say in any electoral matter. But most worrying to Mexicans is the uncertainty and instability that could hang over the country during the weeks the judges sift through evidence and deliberate.
A victory for Lopez Obrador would have been the first time that a true left-wing candidate came to power in Mexico.
After a muddy campaign in which Calderon labeled Lopez Obrador a ''danger to Mexico,'' the country's financial markets rallied as Calderon's win became assured Thursday.
''The contest is behind us. The time for unity and agreement among Mexicans has arrived,'' Calderon said, calling for a government of national unity in a victory speech before several hundred supporters at his party's headquarters.
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THE RESULTS: Ruling party candidate Felipe Calderon defeats leftist Andres Manuel Lopez Obrador in Mexico's presidential race by 0.57 percent, or 243,934 votes.
THE CHALLENGE: Lopez Obrador vows to take the results to Mexico's federal electoral court in an effort to turn the results in his favor, ensuring a nasty battle ahead.
THE ALLEGATIONS: Lopez Obrador alleges widespread fraud and a return to the vote-rigging past. He urges his supporters to fill Mexico City's main square Saturday in a show of force.