Andrés Manuel López Obrador at a rally in Mexico City on Saturday. The candidate is calling for a recount in about 50,000 polling places.

MEXICO CITY (By James C. McKinley Jr., NYTimes) July 10, 2006 — A day after calling for mass demonstrations, Mexico's leftist presidential candidate began a legal battle on Sunday evening to try to overturn the results of the close presidential race.

The candidate, Andrés Manuel López Obrador, a former Mexico City mayor who campaigned on promises to reduce poverty, formally challenged the results in a single election district in the capital that went to the conservative winner of the election, Felipe Calderón. The act was largely symbolic, but his aides said other challenges would be submitted to local election boards and to the Federal Electoral Tribunal before midnight Monday.

Mr. López Obrador's lawyers contend that the initial count of the ballots, on election night a week ago, was careless. In some cases, they also assert that poll workers purposefully inflated Mr. Calderón's totals. Mr. López Obrador is calling for a complete recount in about 50,000 of the 130,000 polling places, where he contends he has evidence of irregularities or fraud.

"They counted the votes, but badly," said Mr. López Obrador, of the Party of the Democratic Revolution. "What we are asking is that they count the votes once again, polling place by polling place."

On Thursday, election officials determined that Mr. Calderón won the election by a narrow margin of 240,000 votes of 41 million cast. Although the Federal Electoral Tribunal has yet to ratify the official tally, Mr. Calderón has declared himself the winner and has received congratulations from the White House, Canada and the European Union.

Mr. López Obrador remains convinced he won and refuses to concede defeat. He is preparing legal challenges to present to both the Federal Electoral Tribunal and the Supreme Court.

At a rally of 150,000 people in the capital's central square on Saturday, he called on his supporters to stage two large marches this week to protest the outcome. Earlier, he told reporters he would mount a sustained campaign of peaceful protests.

As worries grew here that the marches could turn violent over time, the European Commission on Sunday called on both sides to show restraint while the courts dealt with the legal challenges.

The election results contain some oddities that Mr. López Obrador's lawyers hope to use to force a recount or, possibly, to annul the election. There were 900,000 "null votes," in which the voter's intention was unclear to poll workers. Since that number is unusually large and enough to change the outcome, lawyers for Mr. López Obrador's party contend that each ballot should be re-examined.

Second, there were numerous instances during the unofficial tally on Sunday night and Monday morning in which poll workers reported to the electoral institute's headquarters more votes for Mr. Calderón than he had actually received, especially in northern states where the conservative candidate's National Action Party is strong. Mr. López Obrador has said that pattern is evidence of fraud. Mr. Calderón's aides denied the party had committed any wrongdoing.

Finally, during the official tally on Wednesday and Thursday, the chiefs of local election boards uniformly rejected requests from Mr. López Obrador's representatives to recount ballots in thousands of polling places where there were irregularities.

Instead, the election officials stuck to the letter of the electoral law, which requires a recount only if a tally sheet is lost or contains an error or the ballot box appears to have been damaged. The law does not prohibit recounts for other causes, but it does not spell out the conditions either.

In about 2,600 polling places, election officials did recount ballots because of mistakes on tally sheets. Mr. López Obrador pointed out that in a vast majority of those cases, the counts were off.

Martín Ramírez, a spokesman for the electoral tribunal, said the court could not ratify the official tally and recognize Mr. Calderón as president-elect until it had ruled on all of Mr. López Obrador's legal challenges. By law the court must certify a new president by Sept. 6.

By late Sunday afternoon, the tribunal still had not received the official results from the Federal Electoral Institute, Mr. Ramírez said.

John M. Ackerman, an expert on electoral laws at the National Autonomous University of Mexico, said because electoral laws were relatively untested, it was hard to predict Mr. López Obrador's chances of success.

The electoral tribunal was empowered in 1996 to validate presidential elections. The only opportunity it has had to use those powers was in 2000, when President Vicente Fox of the National Action Party handily defeated the candidate of the Institutional Revolutionary Party, ending 71 years of its single-party rule.

Mr. Ackerman said that in the tribunal's short history it had ordered broad, though not complete, recounts in state and local elections. The largest recount, he said, was six years ago in the state of Jalisco, where the race was so tight and fiercely contested that the state was compared to Florida in 2000.

Mr. Ackerman also said the tribunal had not hesitated in the past to reverse decisions by the Federal Electoral Institute.

Mr. López Obrador has proved unpredictable himself. He is known for the demonstrations he staged in 1994 to protest his loss in the governor's race in his home state of Tabasco, charging that the governing party had committed fraud at the polls.

Now he has stepped up his accusations against President Fox, Mr. Calderón and their party, the PAN, saying they are betraying the democracy they helped create and engaging in the same fraudulent practices as the old governing party, the PRI. He also contends that Mr. Fox used state resources to help Mr. Calderón's campaign, a claim both men deny.