CAIRO, Egypt - How do you count 39 million handwritten paper ballots in a matter of hours and declare a winner? That's the 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 reelection the morning after Friday's vote, calling it a "divine assessment."

But yesterday, after two days of street protests, he ordered an investigation into the allegations of fraud.

Mir Hossein Mousavi, Ahmadinejad's reformist challenger, says 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.

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 ratio.

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 ballot papers in the provinces of Fars and East Azerbaijan, where he had been expected to do well because he is among Iran's Azeri minority. Interior Ministry results show Ahmadinejad won in East Azerbaijan.

The final tally was 62.6 percent for Ahmadinejad and 33.75 for Mousavi - a landslide in a race that was perceived to be close.

"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 transparency, 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, which tracked developments in the Iranian vote from outside the country.

He noted that the vote was organized by two institutions that are not independent - the Interior Ministry and the Guardian Council, a 12-member body of clerics and experts in Islamic law closely allied to Iran's supreme leader, Ayatollah Ali Khamenei.

The central question was how 39.2 million paper ballots could be counted by hand and final results announced in Tehran in just over 12 hours. Past elections took at least twice as long.

A new computerized system might have helped speed the process in urban centers, where most Iranians live.

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