A spokesman for the Guardian Council said that the 12-member body would meet the candidates on Saturday, though the body had already begun the "careful examination" of a total of 646 complaints submitted in connection with the June 12 vote.
"This will enable them to raise issues and points they wish to discuss with the members of the council, and also provide a direct contact with the candidates," Abbas-Ali Kadkhodaie said.
Ahmadinejad, who was declared to have won a landslide victory, was also expected to be invited to the meeting. The two other defeated candidates are reformist Mehdi Karoubi and conservative Mohsen Rezaie, a former head of Iran's Revolutionary Guards.
The Guardian Council has already said it may order a partial recount of the ballots, but will not annul the result.
About 100 people gathered outside the United Nations building in Tehran on Thursday calling for the body to take action over the disputed poll.
But Al Jazeera's Alireza Ronaghi, reporting from Tehran, said that a much larger turnout was expected for a later protest at Imam Square.
"I am sure many people will gather and many people will try their best to keep calm and peaceful to commemorate those who lost their lives in the clashes," he said.
Mousavi urged his supporters to wear black as a sign of remembrance and remain peaceful.
"In the course of the past days and as a consequence of illegal and violent encounters with [people protesting] against the outcome of the presidential election, a number of our countrymen were wounded or martyred," he said.
"I ask the people to express their solidarity with the families ... by coming together in mosques or taking part in peaceful demonstrations."
Tens of thousands of anti-Ahmadinejad supporters staged a protest in Tehran for the fifth straight day on Wednesday, despite the authorities' ban on opposition gatherings.
'Lack of confidence'
Baqer Moin, the author of Khomeini: Life of the Ayatollah, told Al Jazeera that the protests were not a movement against the Islamic republic itself.
"I think people are expressing their lack of confidence in the institutions of the Islamic republic and they are asking for a corrective movement so they can trust their institutions," he said.
On Friday, Ayatollah Ali Khameini, the country's supreme leader, will lead prayers at the Tehran University campus.
The organisers have invited all Iranian people and authorities to attend to show solidarity and unity.
Authorities have shut down internet sites and mobile telephone text services in an attempt to bring the demonstrations under control.
Despite these measures, violent scenes of police beating Mousavi supporters taken on mobile phones have been broadcast on news bulletins across the world, though the authenticity of such footage often cannot be verified.