Iranian President Mahmoud Ahmadinejad sought to disperse the clouds of doubt surrounding his re-election in his first major television address. But even as he spoke, opponents went to their rooftops shouting "death to the dictator" — a sign of continuing defiance.

In his half-hour address late Tuesday, Ahmadinejad insisted that the June 12 elections were fair and that his government was legitimate. His staunch line gave no ground to opponents who claim the vote results were fraudulent and launched a wave of mass protests in recent weeks.

"It was the most clean and free election in the world," Ahmadinejad said, adding that during a re-count "no fault was discovered. The whole nation understood this." He said the 85 percent turnout and his landslide victory according to official results had given his government a new legitimacy.

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In a bid to win over skeptics, he promised to accomplish "higher and grander things" during his second term, saying his government would focus on improving the economy. "This is a new beginning for Iran ... we have entered a new era," he said. "We must all join hands to achieve Iran's lofty goals."

Ahmadinejad has the support of Iran's supreme leader Ayatollah Ali Khamenei and hard-liners in the clerical leadership. Khamenei declared the election results valid and unleashed security forces to put down the giant street protests with a wave of arrests.

But while calm has been imposed, Iranians in many parts of the capital continued late Tuesday what has been a nightly ritual of defiance — climbing to their rooftops to shout, "death to the dictator" and "God is great." The shouts could be heard during Ahmadinejad's address.

Opposition leader Mir Hossein Mousavi, who claims to have won the election, is seeking to rekindle the movement after its protest momentum was shattered by the crackdown by police, Revolutionary Guards and Basij militia. Mousavi met late Monday with the other two top pro-reform leaders, former president Mohammad Khatami and cleric Mahdi Karroubi, who also ran in the presidential election.

The three demanded that ruling clerics end the heavy "security atmosphere" imposed after the elections and free those detained in the unrest, according to Mousavi's Web site. They warned that continuing the security crackdown "will only lead to radicalization of political activities."

There was no sign of a let-up in the clampdown. Police say 20 people were killed in postelection violence and more than 1,000 arrested, though they say many have been released.

Authorities this week closed universities and dormitories, apparently because of Web site calls for new protests on Thursday, the anniversary of a 1999 attack by Basij and police on protesting students.

The anniversary could be a test of how willing opposition supporters are to defy tough security measures. The government has also closed government offices, saying the closure is due to unusually heavy dust clouds and pollution hanging over the capital and other parts of the country the past two days.

Mousavi hinted on Monday that he may move away from the tactic of protests and create a political party to work in what he called "a legal framework."

But it is not clear how much margin the opposition will have for political action. Many of the top reform figures — including Khatami's former vice president and one-time members of his Cabinet — are in detention and could face charges of fomenting riots. Earlier this week, the head of the Revolutionary Guards warned that the elite force would take a major role in defending the country's system of clerical rule.

Despite the regime's rhetoric, a number of top clerics have continued to question the election — a rare defiance of the supreme leader from the ranks of the religious establishment.

Six U.N. human rights experts on Tuesday issued a statement expressing "grave concern about reports of killings, ongoing arrests, use of excessive police force and the ill-treatment of detainees." They questioned the legality of the arrests of journalists and demonstrators, saying they face "arbitrary detentions."

Ten Nobel Peace Prize winners including Archbishop Desmond Tutu sent a letter to U.N. Secretary General Ban Ki-moon Tuesday asking him to send a special envoy to Iran to investigate allegations of human rights abuses.

"We deplore the violence and crackdown on peaceful protesters, the increasing restrictions on civil liberties and the imprisonment of ... civic leaders," the letter said.