TEHRAN (Reuters/AFP) - Tens of thousands of Iranians, wearing black and carrying candles, marched on Thursday to mourn those killed in mass protests against a presidential election they and defeated candidate Hossein Mousavi say was rigged. Chanting Allahu Akbar (God is Greatest), they massed in downtown Imam Khomeini Square, responding to Mousavis call for people to gather in mosques or at peaceful rallies to show solidarity with the victims and their families. Iranian state media has reported seven or eight people killed in protests since the election results were published on June 13. Demonstrators held photographs of those killed, some showing bloodied faces, apparently taken after they died. Our martyred brothers we will take back your votes, read one placard. Why did you kill our brothers? said another. Mousavi, a moderate who advocates better ties with the West, addressed people in the crowd with a loudspeaker. He wore a black shirt and suit, witnesses said. Security agents detained opposition politician Ebrahim Yazdi while he was in hospital, an ally of his said. Yazdi heads the banned Freedom Movement and was foreign minister in Irans first government after the revolution. The rally came as the electoral watchdog, the Guardians Council, said it was investigating a total of 646 complaints of irregularities in the conduct of the poll from Mousavi and the other two losing candidates. It said it had invited the trio to set out their grievances on Saturday, with a decision on Sunday about any possible recount in the June 12 election, which returned hardline incumbent Mahmoud Ahmadinejad to power. A spokesman for the Council said Objections include a shortage of ballot papers, pressure on voters to support a particular candidate, and the barring of candidates representatives from polling stations. Mousavi and fellow-candidates Mehdi Karoubi and Mohsen Rezaie could raise their objections at an extraordinary Guardian Council meeting on Saturday, said spokesman Abbasali Kadkhodai. Ahmadinejad supporters are expected to show their strength when Khamenei leads Friday prayers at Tehran University. The semi-official Fars news agency said two children of conservative former President Akbar Hashemi Rafsanjani, who backs Mousavi, had been barred from leaving Iran. Irans Intelligence Ministry said it had uncovered a foreign-linked terrorist plot to plant bombs in mosques and other crowded places in Tehran during the election. State broadcaster IRIB quoted a ministry statement as saying several terrorist groups had been discovered, adding they were linked to Irans foreign enemies, including Israel. In new measures against the media, the elite Revolutionary Guards told websites to delete material that creates tensions or face legal action, and the foreign ministry issued a new warning to the foreign media, accusing some outlets of becoming the mouthpiece of the rioters movement. Pictures, videos and updates from the streets of Iran continue to pour in to social-networking and image-sharing websites such as Twitter, Facebook and YouTube despite Iranian efforts to cut off mobile phones and the Internet. Sometimes jumpy footage broadcast on the Internet from amateur videos has shown chaotic and sometimes brutal scenes of violence, with police beating protesters and one image purportedly showing a protester shot dead during massive street protests on Monday.