At about the same time, a statement on Mousavi's website announced that he had formally appealed against the election result to the legislative body, the Guardian Council.
"Today, I have submitted my official formal request to the council to cancel the election result," he was quoted as saying.
"I urge you Iranian nation to continue your nationwide protests in a peaceful and legal way."
Ahmadinejad was declared the winner by a wide margin, with figures from the interior ministry showing he had taken 62.63 per cent of the vote, while Mousavi garnered only 33.75 per cent.
Ahmadinejad defended the conduct of the election at a news conference at the presidential office in Tehran earlier on Sunday.
He also questioned Mousavi's claims that the vote was "rigged".
"It is not clear how they are questioning the participation of 40 million people in these elections," he said.
"I have still not been shown any documents by anybody, they are just saying the outcome is unexpected.
"It is like football, everybody expects their team to win."
Al Jazeera's Teymoor Nabili, reporting from Tehran, said that Ahmadinejad was not giving too much credence to the idea of a reformist movement taking root.
|Ahmadinejad: It is like football, everybody expects their team to win [EPA]
Noting that the president's analogy was of a football ground, he said: "There may be plenty of people within your field of vision but that doesn't mean that they are the majority of people in the country.
"So he doesn't really seem to think that there is any question that the majority of people within the country are behind his position and his political policies and this election has proved it to him.
"So he doesn't seem to need to reach out or make concessions with anybody who doesn't agree with that."
Robert Fisk, a journalist with the UK's Independent newspaper, told Al Jazeera that Ahmadinejad was repeating the point that the high turnout proved that he was in the majority.
"But many of the people that did vote believe that the vote was switched," he said.
"So it was not the turnout that proved that Ahmadinejad is a popular president. It is what the figures actually were. And that of course is what is still being disputed."
Fisk said one Mousavi supporter had pointed out to him that "if the figures were being counted properly on Friday night, five million votes would have had to have been counted in two hours".
A range of communications has been disrupted inside Iran since election day, including those which could be used to organise protests.
Text messages could not be sent from mobile phones on Sunday, and social networking sites such as Facebook and Twitter were not working.
|Police said 170 people had been detained
over the protests [AFP]
Several foreign news organisations complained that Iranian authorities were blocking their reporters from covering protests against Ahmadinejad's re-election.
German public television channels ZDF and ARD said their reporters were not allowed to broadcast their reports, while the Dubai-based Arab news channel Al-Arabiya said its Tehran office was shut down for a week.
Throughout Sunday, protesters opposing the election results clashed with riot police.
Police were still in the street as night fell and said they had detained 170 people over the protests.
Police have also detained more than 100 reformers, including a brother of Mohammad Khatami, a former president, a leading reformer said.
But a police official denied Khatami's brother had been arrested.
Earlier in the day protesters had set fires and smashed shop windows, while police hit back.
Al Jazeera's Alireza Ronaghi, reporting from Tehran, said the official Islamic Republic news agency was reporting that a committee led by two senior supporters of Mousavi was organising the riots against the election result.
"Whether this is really an honest outburst of anger against the outcome of the election we don't know yet, but what we see is a major crackdown on reformists and their leaders," he said.
Sadegh Zibakalam, head of the Iranian studies department at Tehran University, told Al Jazeera that the demonstrations were largely "spontaneous" responses to the election result.
"No one is giving them commands, no one is ordering them, no one is leading them," he said.
"Nevertheless, the government has started a crackdown on the leading reformist figures ... The government reaction is too harsh, but it is understandable."