Day of mourning
Hundreds of riot police prevented demonstrators from gathering outside parliament as the authorities continued their crackdown on Mousavi's supporters on Wednesday.
Police with riot shields and batons surrounded the area, stopping the several hundreds protesters from getting into the adjoining square.
The attendance was well down on the tens of thousands who attended a series of marches last week.
It was unclear whether a day of mourning for at least 19 people killed since June 12, called for by Mehdi Karroubi, another defeated presidential candidate, would go ahead on Thursday amid a ban on gatherings by opposition groups.
Grand Ayatollah Hossein Ali Montazeri, an Iranian religious leader who has fallen out with the current government, has warned the crackdown could destabilise the government.
"If Iranians cannot talk about their legitimate rights at peaceful gatherings and are instead suppressed, complexities will build up which could possibly uproot the foundations of the government, no matter how powerful," he said in a statement sent to the AFP news agency.
"My recommendation to the great and dear Iranian nation is to pursue its logical and fair demands in complete calm."
Baqer Moin, an Iranian author and journalist, told Al Jazeera that there appeared to be a battle of wills between the two sides in Iran.
"Nobody would like to make any compromise, naturally the opposition leaders rely on the street pressure on the government, but the government has got the advantage of heavy security pressure everywhere," he said.
Meanwhile, Ahmadinejad has demanded that Barack Obama, the US president, stop "interfering" in Iran's affairs, the Fars news agency reported.
"I hope you [Obama] will avoid interfering in Iran's affairs and express regret in a way that the Iranian people are informed of it," he was quoted as saying.
He said that Obama's comments following the disputed election were similar in tone to those of this predecessor, George Bush, and could put an end to any hopes of dialogue between the two countries, which have not had diplomatic relations for 30 years.
"Will you use this language with Iran [in any future dialogue]? If this is your stance, there will be nothing left to talk about. Do you think this behaviour will solve the problem for you? This will not have any result except that the people will consider you somebody similar to Bush."
Ahmadinejad has repeatedly accused the US, Britain and other Western nations of backing the protesters disputing the election result.
Since taking office in January, Obama has made diplomatic overtures towards Iran. But as the unrest has increased he has become incresinly critical.
In his most recent comments, Obama said there were significant questions about the election results and that he was "appalled and outraged" by the violent suppression of the protests.
Mousavi and Akbar Hashemi Rafsanjani, a powerful religious leader and former president, met Iran's national security and foreign policy committee on Thursday to discuss the unrest.
Alaeddin Boroujerdi, the head of the parliamentary committee, told Press TV, an English-language news channel funded by the Iranian government, that the talks were "constructive".
"The lawmakers asked Ayatollah Hashemi Rafsanjani to help solve the problems and he vowed support and we hope that we would witness practical measures to be taken to end the current situation soon," he said.
Rafsanjani - the head of the Assembly of Experts and the Expediency Council, two of Iran's most influential bodies has come under fire from conservatives for his support for Mousavi.
Protesters have said that they plan to continue their protests on Friday by releasing thousands of balloons imprinted with the message "Neda you will always remain in our hearts" - a reference to the young woman killed last week whose image has become an icon of the protests.
Khamenei has vowed not to give in to the protesters.
"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.