Supporters of Mir Hossein Mousavi are planning a new demonstration in Tehran in protest at what they see as a fraudulent presidential poll in Iran, BBC reported.
The planned rally comes after a night in which security forces reportedly raided university dormitories in several Iranian cities.
Iran's Supreme Leader, Ayatollah Ali Khamenei, has sought to calm tensions and called for an end to rioting.
Mahmoud Ahmadinejad was re-elected last week with almost two-thirds of votes.
Protests have grown since his re-election was confirmed on Saturday, with huge demonstrations in Tehran and clashes between protesters and security forces. Eight people have been killed.
The latest opposition demonstration is expected to wind its way though central Tehran, reports the BBC's Jon Leyne from the city.
A similar march on Tuesday is thought to have passed off peacefully, although few details have emerged.
Iran has imposed tough new restrictions on foreign media, requiring journalists to obtain explicit permission before covering any story. Journalists have also been banned from attending or reporting on any unauthorised demonstration.
Ayatollah intervention
Overnight, members of Iran's Basij volunteer militia reportedly raided university dormitories in several Iranian cities.
Students have been active among Iran's opposition and there have been several reports of security forces moving in on university premises since protests began over the weekend.
The raids came after another direct intervention in the crisis by Iran's Supreme Leader Ayatollah Ali Khamenei.
Ayatollah Khamenei has not appeared in public since the election results, but now seems to be deeply involved in the search for a solution to the stand-off.
Meeting representatives of the four election candidates, he urged all parties not to agitate their supporters and stir up an already tense situation.
"In the elections, voters had different tendencies, but they equally believe in the ruling system and support the Islamic Republic," the Associated Press reported him as saying.
"Nobody should take any action that would create tension, and all have to explicitly say they are against tension and riots."
He also repeated his offer of a partial vote recount, a proposal already rejected by the main opposition.
But our correspondent says the ayatollah's personal intervention in the crisis is reducing his political authority.
Obama 'concern'
Widespread anger at the result brought hundreds of thousands of Mr Mousavi's supporters on to the streets on Monday and eight protesters died when a rally ended in violence.
A witness told the BBC that Tuesday's rally in northern Tehran was even bigger than Monday's - though this cannot be independently confirmed - and the state Press TV also described it as large.
Witnesses described demonstrators walking in near silence towards state TV headquarters - apparently anxious not to be depicted as hooligans by authorities.
Thousands of President Mahmoud Ahmadinejad's supporters staged a counter-rally in Vali Asr Square in central Tehran - some bussed in from the provinces, observers say.
A BBC correspondent in Tehran said that protesters also blocked roads with their cars and police set up roadblocks to control gatherings of demonstrators.
As night fell, residents took to the roof-tops of their houses to shout protest messages across the city, a scene not witnessed since the final days of the Shah, our correspondent says.
In the US, President Barack Obama sought on Tuesday to stay neutral in the debate over Iran's election results, insisting he did not want to "meddle" in the affairs of the Islamic Republic.
"It is not productive, given the history of US and Iranian relations to be seen as meddling in Iranian elections," he said.
"But when I see violence directed at peaceful protesters, when i see peaceful dissent being suppressed... it is of concern to me and it is of concern to the American people."
But in a TV interview he also cautioned that there might not be much difference between the policies of President Ahmadinejad and rival Mir Hossein Mousavi.