At a press conference around midnight, Mousavi declared himself "definitely the winner" based on "all indications from all over Iran." He accused the the government of "manipulating the people's vote" to keep Ahmadinejad in power and suggested the reformist camp would stand up to challenge the results.
"It is our duty to defend people's votes. There is no turning back," Mousavi said, alleging widespread irregularities.
There were worries of protests by Mousavi supporters if he is declared the loser, though there was no sign of gatherings Saturday morning. About a dozen supporters sitting on chairs outside one of his campaign offices in central Tehran said they were waiting to hear from their candidate before deciding what action, if any, to take.
Bringing any showdown into the streets would certainly face a swift backlash from security forces. The political chief of the powerful Revolutionary Guard cautioned Wednesday it would crush any "revolution" against the Islamic regime by Mousavi's "green movement."
And it was unclear how many Iranians were even aware of the claims of fraud, amid widespread communications disruptions that began in the later hours of voting Friday — suggesting an information clampdown. State television and radio only broadcast the Interior Ministry's vote count.
Nationwide, the text messaging system remained down Saturday and pro-Mousavi Web sites were blocked or difficult to access. Text messaging is frequently used by many Iranians — especially young Mousavi supporters — to spread election news.
In Tehran's streets Saturday morning, Iranians heading to work gathered around newspaper stands to read the headlines, which did not specifically declare a victor — or carry word of Mousavi's claims.
Mousavi's paper, Kalemeh Sabz, or the Green Word, and other reformist dailies were ordered to change their headlines originally declaring Mousavi the victor, according to editors at the papers, who spoke on condition of anonymity because of the sensitivity of the issue. The papers had blank spots where articles were removed.
The messy and tense outcome capped a long day of voting. It was extended for several hours to accommodate a huge turnout that had people waiting for hours at polling stations in blistering heat and nighttime downpours.
Mousavi, a 1980s-era prime minister, was counting on an outpouring from what's been called his "green tsunami" — the signature color of his campaign and the new banner for reformists seeking wider liberties at home and a gentler face for Iran abroad. He raised hopes that a new leadership might embrace President Barack Obama's invitation to open dialogue and take a less confrontational path with the West over Iran's nuclear program.
The heavy turnout had been expected to help Mousavi. But moments after Mousavi's news conference, Iran's state news agency IRNA reported Ahmadinejad the winner. After what had been seen as a close contest, the overwhelming margin for Ahmadinejad in the Interior Ministry's partial results was startling.
By Saturday morning, Ahmadinejad had 64.7 percent and Mousavi had 32.2 percent with 82 percent of all votes counted, said Kamran Daneshjoo, a senior official with the Interior Ministry, which oversees the voting.
Even in Mousavi's hometown province of Tabriz in northwestern Iran, the ministry claimed Ahmadinejad received more than 60 percent of the vote.
Based on the ministry's figures, around 75 percent of the 46.2 million eligible voters went to the polls.
Mousavi appealed to Iran's supreme leader, Ayatollah Ali Khamenei, to intervene and stop what he said were violations of the law. Khamenei holds ultimate political authority in Iran. "I hope the leader's foresight will bring this to a good end," Mousavi said.
Mousavi said some polling stations were closed early with people still waiting to vote, that voters were prevented from casting ballots and that his observers were expelled from some counting sites.
Authorities "should not assume that by manipulating people's vote and staying in power for a day, for a year or two, (they) can win people's satisfaction," he said.
During the voting, some communications across Iran were disrupted. Internet connections slowed dramatically in some spots, hindering the operations of news organizations including The Associated Press. It was not immediately clear what had caused the disruptions.
About a dozen Ahmadinejad supporters pelted a Mousavi office in Tehran with tear gas canisters, but no one was injured, said Saeed Shariati, head of Mousavi's Web campaign. The attack could not be independently confirmed.