CAIRO — How do you count almost 40 million handwritten paper ballots in a matter of hours and declare a winner? That's a key question in Iran's disputed presidential election.

International polling experts and Iran analysts said the speed of the vote count, coupled with a lack of detailed election data normally released by officials, was fueling suspicion around President Mahmoud Ahmadinejad's landslide victory.

Iran's supreme leader endorsed the hard-line president's re-election the morning after Friday's vote, calling it a "divine assessment" and appearing to close the door on challenges from Iran's reformist camp. But yesterday, after two days of rioting in the streets, he ordered an investigation into the allegations of fraud.

Mir Hossein Mousavi, Ahmadinejad's reformist challenger, claims he was robbed of the presidency and has called for the results to be canceled.

Mousavi's newspaper, Kalemeh Sabz, or the Green Word, reported on its Web site that more than 10 million votes were missing national identification numbers similar to U.S. Social Security numbers, which make the votes "untraceable." It did not say how it knew that information.

Mousavi said some polling stations closed early with voters still in line, and he charged that representatives of his campaign were expelled from polling centers even though each candidate was allowed one observer at each location.

His supporters have reported intimidation by security forces who maintained a strong presence around polling stations.

Observers who questioned the vote said that at each stage of the counting, results released by the Interior Ministry showed Ahmadinejad ahead of Mousavi by a 2-1 margin.

That could be unusual, polling experts noted, because results reported first from Iran's cities would likely reflect a different ratio from those reported later from the countryside, where the populist Ahmadinejad has more support among the poor.

Mousavi said the results also may have been affected by a shortage of ballots in the provinces of Fars and East Azerbaijan, where he had been expected to do well because he is among the country's Azeri minority. Interior Ministry results show that Ahmadinejad won in East Azerbaijan.

The final tally was 62.6 percent of the vote for Ahmadinejad and 33.75 for Mousavi — a landslide victory in a race that was perceived to be much closer. Such a huge margin also went against the expectation that a high turnout — a record 85 percent of Iran's 46.2 million eligible voters — would boost Mousavi, whose campaign energized young people to vote. About a third of the eligible voters were under 30.

Ahmadinejad, who has significant support among the poor and in the countryside, said Sunday that the vote was "real and free" and insisted the results were fair and legitimate.

"Personally, I think that it is entirely possible that Ahmadinejad received more than 50 percent of the vote," said Konstantin Kosten, an expert on Iran with the Berlin-based German Council of Foreign Relations who spent a year from 2005-06 in Iran.

Still, he said, "there must be an examination of the allegations of irregularities, as the German government has called for."

But Iran's electoral system lacks the transparency needed to ensure a fair election, observers said. International monitors are barred from observing Iranian elections and there are no clear mechanisms to accredit domestic observers, said Michael Meyer-Resende, coordinator of the Berlin-based Democracy Reporting International.

One of the central questions was how 39.2 million paper ballots could be counted by hand and final results announced by authorities in Tehran in just over 12 hours.

Each ballot — on which a candidate's name was written in — would have to be counted by hand before any data could be entered into computers, aggregated and transmitted to the Interior Ministry in Tehran.

"I wouldn't say it's completely impossible," Meyer-Resende said. "In the case of Iran, of course, you wonder with logistical challenges whether they could do it so fast."