Opposition protesters took to the streets of the Iranian capital, Tehran, on Sunday in defiance of official warnings of a crackdown.
Police blocked streets leading to the centre of Tehran to try to prevent thousands of people from joining the protests.
It was the worst violence in Iran since the immediate aftermath of June's bitterly disputed presidential election.
Opposition leaders criticised the government for using violence to suppress protests on what was a holy day for Shia Muslims.
Security forces used tear gas to try to disperse protesters but opposition websites said police also fired shots at the crowds.
State-run TV said at least eight people had died. The police denied being responsible for any of the deaths; some of their own officers were hurt in the clashes.
It appears that the scale of the protests took the police by surprise and they lost control of some areas.
Sunday's protests highlight the simmering tension in Iran six months after the disputed presidential elections.
The unrest coincided with the climax of the Shia Muslim festival of Ashura to mourn the 7th Century death of Imam Hussein, a grandson of the Prophet Muhammad.
Given the depth of feeling on the streets, it is unclear how this latest crisis will unfold.
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