Iranian presidential candidate Mir Hossein Mousavi was reportedly arrested Saturday following the reformist's defeat at the polls by hardliner Mahmoud Ahmadinejad. Supporters of Mousavi, the main challenger to Ahmadinejad, have responded to the election with the most serious unrest in Tehran in a decade and claim that the result was the work of a dictatorship.

There have been a number of contradictory reports from Iran, in large part due to the heavy restrictions imposed on the media in the Islamic Republic and in particular on foreign reporters.

Mousavi's arrest was reported by an unofficial source, who said that the presidential contender had been arrested en route to the home of Iran's supreme leader, Ayatollah Ali Khamenei. Pro-reform Mousavi has denounced the election as rigged and vowed he will not accept defeat. He and key aides could not be reached by phone Saturday.

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 from Friday's presidential election was announced.

As night fell Saturday, the rioting and fires continued on the streets of Tehran. The city's cell phone network appeared to be down Saturday night. When users tried to call cell phones, a message appeared saying error in connection. There were also reports of difficulties accessing social networking sites - used by Mousavi to rally supporters.

There was no immediate comment from Iran's Telecommunications Ministry and it did not appear that cell phones were down throughout the country. Residents in several provinces say their service is working.

The result 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.

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 the election. In a message on state TV, he urged the nation to unite behind Ahmadinejad, calling the result a "divine assessment."

"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.

It was also reported Saturday that former Iranian president Ali Akbar Hashemi Rafsanjani, a Mousavi supporter, had resigned from all of his official positions in protest against the results of the election.

'Healthy election'

Ahmadinejad, in a nationally televised victory speech, said that the election process had been "free and healthy," and accused the foreign media of coverage that harms the Iranian people. There was more rioting at night and fires continued to burn on the streets of Tehran.

"People voted for my policies," the conservative president said in his first post-election comment. "It was a free and healthy election," he said, without making direct reference to electoral violation assertions by Mousavi.

As he was speaking, supporters of Mousavi clashed with police in various places in Tehran, chanting anti-Ahmadinejad slogans, witnesses said.

"Everybody should respect people's vote ... we need a calm atmosphere to build the country," Ahmadinejad said. He also took a swipe at his opponents in the election, which was marked by unprecedented mudslinging.

Ahmadinejad, who swept to power in 2005 pledging to revive the values of the 1979 Islamic revolution, accused his opponents of corruption during the campaign. They accused him of lying about the economy, which is suffering from high inflation.

"I named some people during the campaign. I was accused of insulting them, but it is not an insult," he said, referring to his comments about Rafsanjani. "Those who have revolutionary background are not allowed to have extravagant demands," he said.

"All political and propaganda machines abroad and sections inside the country have been mobilized against the nation," he said. "They have launched the heaviest propaganda and psychological war against the Iranian nation. Many global networks continuously worked, employing very complicated methods, that work against our nation and arranged a full-fledged battle against us."

Without mentioning the unrest on the streets, Ahmadinejad proclaimed that a new era has begun in the history of the Iranian nation.

"A bright and glorious future is ahead for the Iranian nation. ... I invite everyone to join me in constructing Iran," he said.

Street battles in Tehran

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.

There were also protests by Mousavi supporers in the southern city of Ahvaz in the oil-rich province of Khuzestan who shouted, "Mousavi, take our votes back!" witnesses said.

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.

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.

Police stormed the headquarters of Iran's largest reformist party and arrested several top reformist leaders, said political activists close to the party.

There was no immediate confirmation from authorities of the raid on the party, the Islamic Iran Participation Front. The activists spoke on condition of anonymity because of the sensitivity of the situation.

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."