Mr Ahmadinejad dismissed Tehran's worst violence in a decade as "not important", comparing it to passions after a football match. He insisted Friday's vote was "real and free" and the results showing his landslide victory were fair and legitimate.
Along Tehran's Vali Asr street - where activists supporting rival candidate Mir Hossein Mousavi held a huge pre-election rally last week - tens of thousands marched in support of Mr Ahmadinejad, waving Iranian flags and shouting his name.
Mr Mousavi sent a letter to the Guardian Council - a powerful clerical group - calling for the election result to be cancelled. He has claimed that he was the real winner.
"Fraud is evident, and review and nullification is requested," said the letter posted on Mr Mousavi's website. Mr Mousavi also met Iran's supreme leader Ayatollah Ali Khamenei to discuss his fraud allegations. Shahab Tabatabaei, a prominent activist in Mr Mousavi's pro-reform camp, said Mr Mousavi called on Khamenei to order cancellation of the election results.
Mr Mousavi earlier released a statement saying voiding the result was the only way to restore public trust. He urged supporters to continue their "civil and lawful" opposition to the result and advised police to stop violence against protesters.
The violence has pushed Iran's Islamic establishment to respond with sweeping measures. It has deployed anti-riot squads around the capital and cut mobile phone messaging and internet sites used by Mr Mousavi's campaign.
There is little chance the youth-driven movement could immediately threaten the pillars of power in Iran - the ruling clerics and the vast network of military and intelligence forces at their command. But their discontent raises the possibility that a sustained and growing backlash could complicate Iran's policies at a pivotal time.
US President Barack Obama has offered to open dialogue after a nearly 30-year diplomatic freeze. Iran also is under growing pressure to make concessions on its nuclear programme or face possible more international sanctions.
US Vice-President Joe Biden said today that he had doubts about whether the election was free and fair. He said the US and other countries needed more time to analyse the results before making a better judgment.
In Paris, French foreign minister Bernard Kouchner said his country was "very worried" about the situation in Iran and criticised the authorities' "somewhat brutal reaction" to the protests.
Mahdi Karroubi, a moderate former parliament speaker who also ran in the election, also challenged the result.
"The results for the election are illegitimate and the government lacks national dignity and social competence," he said in a statement. "So I do not recognise Mahmoud Ahmadinejad as president of Iran."
In a second day of clashes, scores of young people shouted "Death to the dictator" and broke the windows of city buses on several streets in central Tehran. They burned banks and set fire to rubbish bins and piles of tyres, using them as flaming barricades to block police.
Riot police beat some of the protesters with batons while dozens of others holding shields and motorcycles stood guard nearby. Shops, government offices and businesses closed early as tension mounted.
Mr Ahmadinejad called the level of violence "not important from my point of view", speaking at a news conference.
"Some believed they would win, and then they got angry," he said. "It has no legal credibility. It is like the passions after a football match. The margin between my votes and the others is too much and no-one can question it."
"In Iran, the election was a real and free one," said Mr Ahmadinejad. "The election will improve the nation's power and its future," he told a packed room of Iranian and foreign media.
Mr Ahmadinejad accused foreign media of launching a "psychological war" against the country, repeating a charge he also made yesterday.
Iranian authorities have asked some foreign journalists - in Iran to cover the elections - to prepare to leave.
Nabil Khatib, executive news editor for Dubai-based news network Al Arabiya, said the station's correspondent in Tehran was given a verbal order from Iranian authorities today that the office will be closed for one week. No reason was given for the order, but the station was warned several times yesterday that it needed to be careful in reporting "chaos" accurately.
Iran restored mobile phone services that had been down in the capital since yesterday. But Iranians could not send text messages from their phones, and the government increased its internet filtering in an apparent attempt to undercut liberal voices. Social networking sites including Facebook and Twitter were also not working.
The restrictions were likely intended to prevent Mr Mousavi's supporters from organising large-scale protests. But smaller groups assembled around the city, and about 300 Mousavi supporters gathered outside Sharif University, chanting "Where are our votes?"
About a dozen riot police used batons to disperse about 50 Mousavi supporters standing outside his campaign quarters.
Yesterday, Mr Mousavi, a 67-year-old former prime minister, released an internet message saying he would not "surrender to this manipulation". Authorities responded with targeted detentions, apparently designed to rattle the leadership of Mr Mousavi's "green" movement - the trademark colour of his campaign.
The detentions include the brother of former reformist president Mohammad Khatami and two top organisers of Iran's largest reformist party, the Islamic Iran Participation Front: the party's secretary-general and the head of Mr Mousavi's youth internet campaign. Mohammad Reza Khatami and the two party activists were released today.
Several others linked to Mr Mousavi's campaign remained in custody, but the full extent of the arrests were not known.
Tehran deputy prosecutor Mahmoud Slarkia told the semi-official ISNA news agency that fewer than 10 people were arrested on the charge of "disturbing public opinion" through their "false reports" on websites after the election. He did not mention any names.
Iran's deputy police chief, Ahmad Reza Radan, told the official Islamic Republic News Agency that about 170 people have been arrested for their involvement in yesterday's protests. He said 10 of those arrested were "main planners" and 50 were "rioters". The others were arrested for being at the site of the clashes, he said. Some of the detained were active in Mr Mousavi's campaign headquarters or had relations with foreign media, he said.
"Police will not allow protesters to disturb the peace and calmness of the people under the influence of foreign media," he said.
Mr Mousavi's newspaper, Kalemeh Sabz, or the Green Word, did not appear on newsstands today. An editor said the paper never left the printing house because authorities were upset with Mr Mousavi's statements.
The paper's website reported that more than 10 million votes in Friday's election were missing national identification numbers, making the votes "untraceable". It did not say how it knew that information.
"Don't worry about freedom in Iran," Mr Ahmadinejad told the news conference. "Newspapers come and go and reappear. Don't worry about it."
Iran's supreme leader, Ayatollah Ali Khamenei, has closed the door for a possible compromise. He could have used his near-limitless powers to intervene in the election dispute. But, in a message on state TV yesterday, he urged the nation to unite behind Mr Ahmadinejad, calling the result a "divine assessment".
