The nephew of Iranian opposition leader Mir Hossein Mousavi is reportedly among eight people killed in continuing clashes between police and protesters.
An aide to the leader said on Sunday that Seyyed Ali Mousavi died after being shot by the police, but the claim could not be independently verified as foreign news organisations are barred by the authorities from covering street unrest.
Iranian state television, however, confirmed that several people were killed in clashes.
Opposition websites said at least four protesters had been killed in the capital Tehran while another four died in a violent crackdown on opposition protests in the northwestern city of Tabriz.
"During clashes between security forces and protesters ... at least four protesters were killed in Tabriz and many others wounded," the Jaras website said.
It earlier said that four protesters had died in Tehran and protesters chanted angry slogans seeking revenge.
"We will kill those who killed our brothers," it quoted demonstrators as chanting.
Pictures linked on the Twitter microblogging site appeared to show a man with a wound to his head being carried away by opposition protesters.
The clashes came as the country's Shia Muslims marked Ashoura, a religious event commemorating the 7th century death of Prophet Muhammad's grandson.
Amateur video footage posted on the internet, said to be from the protest, showed protesters running away from riot police or Basij militias on motorbikes.
Al Jazeera's Mohammad Hassan al-Bahrani, reporting from Tehran, said the police had arrested a number of protesters.
"Hundreds of supporters of the Iranian opposition, mainly of reformist forces, have protested and chanted slogans supporting [Grand Ayatollah Hossein Ali] Montazeri and Mir Hossein Moussavi," he said.
"The police had earlier threatened to face any unlicensed gatherings, and this is what really happened on Saturday and Sunday."
The opposition had urged people to gather in central Tehran on Sunday morning in defiance of warnings of a harsh crackdown on any protests during Ashoura.
There were also unconfirmed reports of fierce clashes in the city of Isfahan and Najafabad.
Ghanbar Naderi, a journalist with the Iran Daily, an official government newspaper, criticised the protesters for choosing the day of Ashoura to stage demonstrations.
"This is the wrong place and the wrong time for the reformist camps to ask their supporters to get to the streets because this is a very important religious day for the whole of the nation," he told Al Jazeera.
"If they are going to challenge the system now, they have no place among the ordinary citizens. People want to mark and commemorate the martyrdom of Imam Hussein and not their support of any political group or faction."
Sadegh Zibakalam, a professor at the University of Tehran, told Al Jazeera that the sheer number of people gathering in the street for religious commemorations would make it difficult for the police to keep control.
"It's very difficult for the police to prevent the crowds today because Ashoura is the most important day in Shia religion. Hundreds and thousands and millions of people come out in the street towards noon.
"It will be very difficult for the police to distinguish between genuine mourners and those who want to use the procession to register their protest against the government."
The latest protests came after violence broke out on Saturday when a speech by Mohammed Khatami, the former president, was cancelled.
Widespread protests have been held in Iran following the disputed presidential election on June 12.
Protests against the re-election of Mahmoud Ahmadinejad triggered deadly clashes between protesters, security forces and government-backed militia.
Opposition leaders Mir Hossein Mousavi and Mehdi Kharroubi have said the election was rigged. (Al Jazeera)
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