Iran's supreme leader, Ayatollah Ali Khamenei, closed the door on any chance he could use his limitless powers to intervene in the disputes from Friday's election. In a message on state TV, he urged the nation to unite behind Ahmadinejad, calling the result a "divine assessment."
But Ahmadinejad's main challenger, Mir Hossein Mousavi, has rejected the result as rigged and urged his supporters to resist a government of "lies and dictatorship."
The clashes in central Tehran were the more serious disturbances in the capital since student-led protests in 1999 and showed the potential for the showdown over the vote to spill over into further violence and challenges to the Islamic establishment.
Several hundred demonstrators — many wearing the trademark green colors of Mousavi's campaign — chanted "the government lied to the people" and gathered near the Interior Ministry as the final count was announced. It gave 62.6 percent of the vote to Ahmadinejad and 33.75 to Mousavi, who served as prime minister in the 1980s and has become the hero of a youth-driven movement seeking greater liberties and a gentler face for Iran abroad.
The turnout was a record 85 percent of Iran's 46.2 million eligible voters. Two other candidates received only a fraction of the vote.
Protesters set fire to tires outside the Interior Ministry and anti-riot police fought back with clubs and smashed cars. An Associated Press photographer saw a plainclothes security official beating a woman with his truncheon.
In another main street of Tehran, some 300 young people blocked the avenue by forming a human chain and chanted "Ahmadi, shame on you. Leave the government alone."
Mousavi's campaign headquarters urged people to show restraint.
Interior Minister Sadeq Mahsouli, who supervised the elections and heads the nation's police forces, warned people not to join any "unauthorized gatherings." Earlier, the powerful Revolutionary Guard said it would not tolerate any challenges by Mousavi's "green" movement — the color adopted by Mousavi's campaign.
"I'm warning that I won't surrender to this manipulation," said a statement on Mousavi's Web site. "The outcome of what we've seen from the performance of officials ... is nothing but shaking the pillars of the Islamic Republic of Iran's sacred system and governance of lies and dictatorship."
He warned "people won't respect those who take power through fraud."
The headline on one of Mousavi's Web sites: "I wont give in to this dangerous manipulation." Mousavi and key aides could not be reached by phone.
It was even unclear how many Iranians were even aware of Mousavi's claims of fraud. Communications disruptions began in the later hours of voting Friday — suggesting an information clampdown. State television and radio only broadcast the Interior Ministry's vote count and not Mousavi's midnight press conference.
Nationwide, the text messaging system remained down Saturday and several 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.
At Tehran University — the site of the last major anti-regime unrest in Tehran in 1999 — the academic year was winding down and there was no sign of pro-Mousavi crowds. But university exams, scheduled to begin Saturday, were postponed until next month around the country.
The state-run Islamic Republic News Agency reported that Ahmadinejad plans a public address later Saturday in Tehran.
Even before the count began, Mousavi declared himself "definitely the winner" based on "all indications from all over Iran." He accused 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.
Mousavi's backers were stunned at the Interior Ministry's results after widespread predictions of a close race — or even a slight edge to Mousavi.
"Many Iranians went to the people because they wanted to bring change. Almost everybody I know voted for Mousavi but Ahmadinejad is being declared the winner. The government announcement is nothing but widespread fraud. It is very, very disappointing. I'll never ever again vote in Iran," said Mousavi supporter Nasser Amiri, a hospital clerk in Tehran.
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."
The Revolutionary Guard is directly under the control of the ruling clerics and has vast influence in every corner of the country through a network of volunteer militias.
In Tehran, several Ahmadinejad supporters cruised the streets waving Iranian flags out of car windows and shouting "Mousavi is dead!"
Mousavi appealed directly to Iran's supreme leader, 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.
Iran does not allow international election monitors. During the 2005 election, when Ahmadinejad won the presidency, there were some allegations of vote rigging from losers, but the claims were never investigated.
The outcome will not sharply alter Iran's main policies or sway major decisions, such as possible talks with Washington or nuclear policies. Those crucial issues rest with the ruling clerics headed by the unelected Khamenei.
But the election focused on what the office can influence: boosting Iran's sinking economy, pressing for greater media and political freedoms, and being Iran's main envoy to the world.
Before the vote count, President Barack Obama said the "robust debate" during the campaign suggests a possibility of change in Iran, which is under intense international pressure over its nuclear program. There has been no comment from Washington since Ahmadinejad was declared the winner.
In Israel, the deputy foreign minister, Danny Ayalon, said "the re-election of Ahmadinejad demonstrates the increasing Iranian threat."
Former President Jimmy Carter said he expects no major change in Iran's policies.
"I think this election has bought out a lot of opposition to his policies in Iran, and I'm sure he'll listen to those opinions and hopefully moderate his position," said Carter after meeting with Palestinian Prime Minister Salam Fayyad in the West Bank city of Ramallah.
