At least 10 people were reported to have been killed in clashes between protesters and police and militia forces on Saturday.
That violence followed a warning on Friday from Iran's Supreme Leader, Ayatollah Ali Khamenei, that further demonstrations against the election result would not be tolerated.
Iranian state media said 457 people were detained over Saturday's violence.
International campaign group Reporters Without Borders says 23 local journalists and bloggers have been arrested over the past week.
Opposition supporters passing messages online said they planned to carry candles at a rally in Tehran on Monday evening in memory of those killed. However, heavy security on the streets meant it was not clear whether they would be able to gather freely.
The protests began after incumbent President Mahmoud Ahmadinejad was named as the winner of the presidential election of 12 June.
Results showed Mr Ahmadinejad won the election by a landslide, taking 63% of the vote, almost double that of Mr Mousavi, his nearest rival.
Following complaints, the powerful Guardian Council, which oversees the electoral process, now says it has found evidence that more votes were cast in some constituencies than there were registered voters. But the number "has no effect on the result of the elections", a council spokesman said.
The BBC's Middle East editor, Jeremy Bowen, who is in Tehran, says this crisis has highlighted divisions within Iran's ruling elite.
Mr Mousavi saying the Islamic Republic needs root and branch reform, and it is hard to imagine Iran being the same place at the end of this crisis, our correspondent says.
'Contacting the enemy'
Speaking at a news conference on Monday, foreign ministry spokesman Hassan Qashqavi accused Western governments of explicitly backing violent protests aimed at undermining the stability of Iran's Islamic Republic.
"Spreading anarchy and vandalism by Western powers and also Western media... these are not at all accepted," he said.
Jeremy Bowen BBC Middle East editor, Tehran
Mr Mousavi needs to find a way of channelling his popular support and to exploit the backing he also has amongst some of Iran's senior clerics and politicians.
At the same time though, the supreme leader and the president are determined to hold to their position. The question for the opposition is how to get that energy coming off the streets to make inroads with the elders at the top level. This is a very tight-knit group of people who have been together for 30 years or more.
What makes this different and unique is that in the past, although there has been unrest, there has never been a break in the elite.
He said the West was acting in an "anti-democratic" manner, instead praising Iran's commitment to democracy and stressing once again that the results of the presidential election were unimpeachable.
Iran has strongly criticised the US and UK governments in recent days, and Mr Qashqavi reserved special scorn for the BBC and for the Voice of America network, which he called "government channels".
The BBC and other foreign media have been reporting from Iran under severe restrictions for the past week. The BBC's permanent correspondent in Iran, Jon Leyne, was asked to leave the country on Sunday.
"They [the BBC and the VOA] are the mouthpiece of their government's public diplomacy," Mr Qashqavi said.
"They have two guidelines regarding Iran. One is to intensify ethnical and racial rifts within Iran and secondly to disintegrate the Iranian territories."
"Any contact with these channels, under any pretext or in any form, means contacting the enemy of the Iranian nation.
"How can they say they are unbiased when their TV channel is like a war headquarters and in fact they are blatantly commanding riots. Therefore their claims are absolutely wrong. Their governments have ratified decisions so that they can act in this way."
Amateur video shows fierce clashes
Witnesses said there were no rallies in the capital on Sunday, a day after 10 people were reported killed in clashes between police and protesters.
Meanwhile, Mr Mousavi, whose supporters make up most of the protesting crowds, urged them to continue their rallies.
"Protesting against lies and fraud is your right. In your protests continue to show restraint," a statement on his website said.
On Sunday, thousands of security officers were out on the streets but protesters stayed away.
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