Earlier during the day, Ayatollah Ali Khamenei, Iran's supreme leader, vowed not to give in to opposition protests over the poll outcome.
The protesters have been challenging the re-election of Mahmoud Ahmadinejad, the president.
"On the current situation, I was insisting and will insist on implementation of the law. That means we will not go one step beyond the law," Khamenei said on state television.
"Neither the establishment nor the nation will yield to pressure at any cost," he said, defending Ahmadinejad's landslide victory.
Mousavi's wife has said that protesters are refusing to buckle under the situation.
Zahra Rahnavard, a former university dean who campaigned beside her husband, said on a pro-Mousavi website that his followers had the constitutional right to protest.
Rahnavard said the government should not deal with them "as if martial law has been imposed in the streets".
Mehdi Karoubi, another defeated candidate in the election, has dismissed Khamenei's warning, calling the new government "illegitimate".
"I do not accept the result and therefore consider as illegitimate the new government. Because of the irregularities, the vote should be annulled," Karoubi's website quoted him as saying.
At least 19 people have been killed in clashes with police and pro-government militia amid angry protests that broke out after Mousavi declared that the election had been rigged.
Karoubi, who came fourth in the poll according to official results, has called for Iranians to hold ceremonies on Thursday to mourn those killed in the protests.
Although streets protests have diminished since police and pro-government militias used tear gas, batons and water cannon against protesters on Saturday, calls for further protests among supporters of Ahmadinejad's opponents have continued.
Nazenin Ansari, the diplomatic editor of the Kahylan newspaper, told Al Jazeera that the fall in numbers gathering to protest was understandable given the "degree of repression on the streets".
Ansari said: "Without a doubt, although there are not millions gathering on the streets because of the indiscriminate fire and repression, this is going to transform [politics in Iran].
"In provinces, where people were before gathering in universities, in recent days we are seeing people gathering in main squares."
Sadeq Mahsouli, Iran's interior minister, has accused the US Central Intelligence Agency (CIA) of helping those who have taken part in the protests.
Mousavi, a former prime minister, and the two other candidates in the election have all filed complaints to the Guardian Council about alleged problems with the June 12 vote.
But on Wednesday, Mohsen Rezaie, the conservative candidate who finished third in the election, withdrew his objections.
"I see it as my responsibility to encourage myself and others to control the current situation," the official IRNA news agency reported Rezaie as saying in a letter to the Guardian Council.
"Therefore I announce that I'm withdrawing my submitted complaints," the former head of the Revolutionary Guard said.
Rezaie had originally complained that he had won more votes than he had been credited with when the interior ministry declared the results.
Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said: "I think he wants to remain in the framework of the Islamic republic - the framework that conservative newspapers are trying to push Mousavi and Karroubi out of.
"Mohsen Rezaie intends to stay close to the core of the Islamic republic and show his allegiance to the supreme leader by obeying his call that the elections are over."
'No major fraud'
Despite Khamenei agreeing to extend the deadline for filing election complaints by five days, a spokesman for the Guardian Council said that there will not be a fresh vote.
"If a major breach occurs in an election, the Guardian Council may annul the votes that come out of a particular affected ballot box, polling station, district or city," Abbas Ali Kadkhodaei was quoted as saying by Press TV, an Iranian government-funded television station.
"Fortunately, in the recent presidential election, we found no witness of major fraud or breach in the election.
"Therefore, there is no possibility of an annulment taking place," he said.
Meanwhile, an aide to Mousavi said that the former prime minister's newspaper had been raided and 25 employees arrested.
Ali Reza Beheshti said that the raid took place on Monday evening as they were preparing to relaunch Kalemeh Sabz, or the Green Word.
The newspaper had been unavailable for more than a week.