Defeated reformist presidential candidate Mir Hossein Mousavi's supporters marched for a national day of mourning for those killed in post-election clashes in a bid to keep up pressure on the authorities to repeat the vote in the country's biggest crisis since the 1979 revolution.
Witnesses said Imam Khomeini Square was packed with people dressed in black and holding candles, a day after Mousavi called on his supporters to gather in mosques or at peaceful rallies to show solidarity with the victims and their families.
"Where are our brothers?" read one banner in the crowd. "Why did you kill our brothers?" read another.
Iran's English-language state television has reported eight people killed in protests since official results from Friday's poll showed President Mahmoud Ahmadinejad had been re-elected.
State media said seven people were killed in an opposition protest in Tehran against what Mousavi says was a rigged election last week in favor of hard line incumbent president, Mahmoud Ahmadinejad.
Iran arrested two top elderly dissidents in a massive crackdown, a press report said on Thursday.
Ebrahim Yazdi and Mohammad Tavasoli, veteran revolutionaries and leaders of Iran's Liberation Movement were arrested on Wednesday, the Etemad Melli newspaper reported.
Unofficial reports said Yazdi had been detained at a hospital emergency unit.
On his website, Mousavi called on Iranians to stage peaceful demonstrations or gather in mosques on Thursday following a "silent" protest rally held Wednesday by tens of thousands of supporters.
Iranian state television broadcast brief footage of the rally while foreign media, banned from reporting on the protests, relied on videos sent by users directly or via YouTube videos.
Grappling with the biggest wave of public anger in three decades of Islamic rule, Iranian authorities lashed out at enemy "plots," hauling in foreign ambassadors and rounding up scores of reformists.
The Islamic Republic accused foreign media of being a "mouthpiece for rioters" and threatened legal action against websites that publish material that "creates tensions" and issued a new warning to the foreign media, already facing tight restrictions on their work.
The media watchdog Reporters Without Borders said a dozen Iranian journalists and bloggers have been arrested and many others have gone into hiding.