More mass protests are expected in the Iranian capital after presidential challenger Mir Hossein Mousavi urged followers to observe a day of mourning, BBC reported.
Mr Mousavi called on supporters to stage peaceful protests or gather in mosques in memory of eight people killed after a Tehran rally on Monday.
The BBC's Jon Leyne, in Tehran, says the government so far has no clear response to the daily demonstrations.
Mr Mousavi is calling for a rerun of last week's presidential election.
President Mahmoud Ahmadinejad was re-elected with almost two-thirds of the votes, but the opposition claims there was widespread fraud.
Mr Mousavi is urging his followers to wear black in Thursday's protests in mourning for those shot by members of the pro-government Basij volunteer militia on Monday.
Heavy restrictions have been placed on the BBC and other foreign news organisations. Reporters are not allowed to cover unauthorised gatherings or move around freely in Tehran - but there are no controls over what they can write or say.
The gatherings are expected to draw even larger numbers than a rally on Wednesday, which saw tens of thousands of people march in silence through the centre of Tehran.
Estimates put the number of protesters at between 70,000 and 500,000.
Mr Mousavi's call for further action on Thursday is in open defiance of Supreme Leader Ayatollah Ali Khamenei, who has urged the nation to unite behind the Islamic state.
Authorities have promised a recount of the disputed votes - but the opposition is demanding the election be held again.
Meanwhile, the Iranian government complained to foreign ambassadors on Wednesday about what it called "meddlesome" and "impertinent" comments made about Iran's internal affairs.
Among those summoned to the foreign ministry was the Swiss envoy who represents US interests in Iran. Iranian officials complained about Washington's "interventionist approach" on the election issue, but the White House has denied the accusation.
Iranian authorities have also rounded up pro-reform figures and tried to further muzzle web sites and social networks used by opposition supporters to broadcast information and images of events in Tehran.
Among those reportedly detained on Wednesday were newspaper editor Saeed Laylaz and Hamid Reza Jalaipour, an activist and journalist.
Members of the Basij militia have also reportedly raided university dormitories in several Iranian cities, ransacking dormitories and beating up some students.
Act of defiance
In another show of defiance on Wednesday, six footballers playing for Iran's national team appeared in a World Cup qualifier in Seoul, South Korea, wearing armbands in the green associated with Mr Mousavi.
Iranian affairs analyst Meir Javedanfar said the protests had forced Ayatollah Ali Khamenei into the centre of an escalating crisis and had broken taboos about questioning his final word on important matters.
"It's changing the way Iranians see the supreme leader and the system in general. That opens up the system up in ways it's never faced before."
The BBC's Jon Leyne says the disputed election has stirred up strong feelings in a population frustrated about a government and a system that fails to meet their aspirations.