"I ask the people to express their solidarity with the families ... by coming together in mosques or taking part in peaceful demonstrations."
The first rally was scheduled for 2pm (09:30GMT) in front of the offices of the United Nations and the second at 4pm at Imam Square, according to people who took part in marches on Wednesday.
Ahmadinejad was officially declared winner of Friday's election by a margin of two-to-one over Mousavi, but the reformist former prime minister has complained that the poll was "rigged".
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.
They marched towards the Vali Asr square, protesting against what they say was a rigged election.
Many were wearing green, the colour of Mousavi's campaign, but Al Jazeera's Alireza Ronaghi, reporting from Tehran, said he had spotted a new trend in their outfits.
"I saw that it was becoming a prevalent trend to wear black ribbons as well, which is a sign of mourning in Iran, a sign of sympathy for the victims who have died in protests the days before," he said.
The protests continued despite the Guardian Council, Iran's most senior legislative body, saying it could order a partial vote recount, provided it finds irregularities.
But the council ruled out annulling the disputed poll, the main demand of the opposition.
Authorities broadened their media clampdown on Wednesday in an attempt to bring the demonstrations under control.
A dozen Iranian journalists and bloggers have been arrested in the aftermath of the contested presidential election, according to Reporters Without Borders, and the government has placed strict restrictions on foreign media coverage, accusing some foreign media of being the "mouthpiece of rioters".
"Some countries, in an uncalculated, hasty and rude reaction towards the illegal gatherings, have supported them contrary to democratic principles and regulations and have become the mouthpiece of the rioters' movement", the foreign ministry said in a statement on Wednesday.
Several internet sites such as Facebook, Twitter and YouTube have been blocked and the Revolutionary Guard, an elite military force, warned the country's online media users that the faced legal action if their uploads "creates tensions".
Al Jazeera's correspondent said the move against websites and blogging by the Guard was beyond their remit.
"Their move to crack down on websites and blogs is against their constitutional rights, but they see things spreading out of hand, so they feel it necessary to intervene at this point," Ronaghi said.
"It is obvious that the Revolutionary Guard does not see itself as a pure military organisation.
"They have been telling the media and activists that the Revolutionary Guard was founded as a political and military foundation of the Islamic Republic, so they see it as appropriate to enter politics whenever they deem fit."
Within the country, mobile phone text services have been down since the election
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.
Also on Wednesday, Iran accused the US of meddling in the crisis over the disputed election results.
|Iran has accused the US of interference, a charge Washington denies [GALLO/GETTY]
A statement by the state-run Press TV blamed Washington for "intolerable interference" and the government summoned the Swiss ambassador, who represents US interests in Iran as Washington and Tehran severed ties in 1979.
A US state department spokesman said Washington was not interfering in Iran's internal affairs.
But it did asked the Twitter networking website to postpone a scheduled maintenance shutdown of its service this week to keep information flowing from inside Iran, three US officials said on condition of anonymity.
Twitter, however, told Al Jazeera that US government pressure had not contributed significantly to its decision to delay the maintenance shutdown.
Barack Obama, the US president, has said the world was inspired by the "amazing ferment" in Iran, but added that there appeared to be little difference in policy between Ahmadinejad and Mousavi.
"Either way we are going to be dealing with an Iranian regime that has historically been hostile to the United States," he said.