His speech came as the country's top three reformist leaders sought to rekindle their opposition movement, demanding that ruling clerics end the heavy "security atmosphere" imposed after the elections and free those detained in the unrest, according to an opposition website.
It was Ahmadinejad's first national speech since the supreme leader declared the election results valid despite outcry from the other candidates and weeks of street protests claiming that the results were fraudulent.
"This is a new beginning for Iran ... we have entered a new era," the president said, explaining that the 85 percent turnout and overwhelming win had given his government a new legitimacy.
"It was the most clean and free election in the world," he said, adding that during the recount "no fault was discovered. The whole nation understood this."
"This election has doubled the dignity of the Iranian nation," he said.
During the half-hour speech, Iranians in many parts of the capital Tehran could be heard shouting from their rooftops, "death to the dictator" and "God is great" -- actions that have become a symbol of defiance since the elections.
Opposition leader Mir Hossein Mousavi, who claims to have won the June 12 election, is struggling for a way to channel the widespread discontent since the vote but which has since been shattered by the harsh crackdown by police, Revolutionary Guards and Basij militia.
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." Late Monday, he met with the other top stars of the reform movement -- former president Mohammad Khatami and Mahdi Karroubi, another election candidate -- in a show of unity.
The three warned Iran's clerical leadership that if the security crackdown continues, it "will only lead to radicalization of political activities," Mousavi's website reported on Tuesday.
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.
There was no sign of a let-up in the clampdown imposed since Iran's supreme leader Ayatollah Ali Khamenei declared the official election results valid and Ahmadinejad the victor.
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 website calls for new protests on Thursday, the anniversary of a 1999 attack by Basij and police on protesting students. It is unclear if anyone will attempt a march -- not only because of the security measures but also because of heavy dust clouds and pollution hanging over the capital and other parts of the country the past two days, forcing the closure of government offices.
French President Nicolas Sarkozy, meanwhile, demanded the release of a young French academic detained after taking photos of Iranian protests and accused of espionage.
Clotilde Reiss, 23, was arrested last week at Tehran's airport as she was about to leave Iran after a five-month stay during which she taught French at Isfahan University.
"These accusations of espionage are high fantasy," Sarkozy said at a news conference Tuesday.
Iran's hard-line leaders have been trying to erase any lingering doubts about the legitimacy of Ahmadinejad's government by portraying the unrest as sparked by foreign meddling.
In his speech Tuesday, the president criticized his election rivals and accused them of working with Iran's enemies.
"Unfortunately, some people inside Iran collaborated with them. They repeated the remarks made by certain Western countries," Ahmadinejad said, as he accused the West of interfering in the country's politics.
"The result of their childish acts of interference in Iran's internal affairs is that the Iranian nation and government will enter the global stage several times more powerful," he said.
Also Tuesday, six U.N. human rights experts 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.
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.
This week, a party close to one of the most politically powerful clerics -- former president Akbar Hashemi Rafsanjani -- issued a statement rejecting Ahmadinejad's victory. It was one of the strongest hints yet on the powerful cleric's stance. He is the head of two major clerical ruling bodies and is a bitter enemy of Ahmadinejad, but has kept his distance from the postelection turmoil.
"Due to the unhealthy trend of the election, widespread irregularities and the support extended by a majority of Guardian Council members to a specific candidate, the result of this election is not acceptable," the Kargozaran party said in its statement, published on Mousavi's website.
AP correspondent Elaine Ganley in Paris and Ron DePasquale at the United Nations contributed to this report.