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."
Several hundred demonstrators - many wearing the trademark green colors of pro-reform candidate Mir Hossein Mousavi's campaign - chanted "the government lied to the people" and gathered near the Interior Ministry as the final count from Friday's presidential election was announced.
It gave 62.6 percent of the vote to Ahmadinejad and 33.75 to Mousavi - a former prime minister who has become the hero of a youth-driven movement seeking greater liberties and a gentler face for Iran abroad.
Mousavi rejected the result as rigged and urged his supporters to resist a government of "lies and dictatorship."
"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," it added.
Mousavi warned "people won't respect those who take power through fraud." The headline on one of his Web sites read: "I won't give in to this dangerous manipulation."
Mousavi appealed directly to Khamenei to intervene and stop what he said were violations of the law. Khamenei, who is not elected, holds ultimate political authority in Iran and controls all major policy decisions.
Mousavi and key aides could not be reached by phone.
U.S. Secretary of State Hillary Rodham Clinton said the U.S. hopes the outcome of the election reflects the "genuine will and desire" of the Iranian people. White House press secretary Robert Gibbs said the U.S. administration is paying close attention to reports of alleged election irregularities.
At a joint appearance with Clinton, Canadian Foreign Affairs Minister Lawrence Cannon said his country was "deeply concerned" by reports of irregularities in the election.
The clashes in central Tehran were the more serious disturbances in the capital since student-led protests in 1999. They showed the potential for the showdown to spill over into further violence and challenges to the Islamic establishment.
The demonstrations began Saturday morning shortly before the government announced the final results.
Protesters set fire to tires outside the Interior Ministry and anti-riot police fought back with clubs and smashed cars. Helmeted police on foot and others on buzzing motorcycles chased bands of protesters roaming the streets pumping their fists in the air. Officers beat protesters with swift blows from their truncheons and kicks with their boots. Some of the demonstrators grouped together to charge back at police, hurling stones.
Plumes of dark smoke streaked over the city, as burning barricades of tires and garbage bins glowed orange in the streets. Protesters also torched an empty bus, engulfing it in flames on a Tehran street.
An Associated Press photographer saw a plainclothes security official beating a woman with his truncheon. Italian state TV RAI said one of its crews was caught in the clashes in front Mousavi's headquarters. Their Iranian interpreter was beaten with clubs by riot police and officers confiscated the cameraman's tapes, the station said.
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." There was no word on any casualties from the unrest.
“I'm warning that I won't surrender to this manipulation.”Statement from Mir Hossein Mousavi's website
It was not clear 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 news 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.
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."
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.
Even before the vote counting 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," he said, alleging widespread irregularities.
Mousavi's backers were stunned at the Interior Ministry's claim that Ahmadinejad won after widespread predictions of a close race - or even a slight edge for the reformist candidate.
Turnout was a record 85 percent of the 46.2 million eligible voters.
"Many Iranians went to the people because they wanted to bring change," said Mousavi supporter Nasser Amiri, a hospital clerk in Tehran. "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."
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.
Ahmadinejad planned a public address later Saturday in Tehran.
In the capital, several Ahmadinejad supporters cruised the streets waving Iranian flags out of car windows and shouting "Mousavi is dead!"
The election 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 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.
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.
(Copyright 2009 by The Associated Press. All Rights Reserved.)