At least 10 people were killed and more than 100 injured during yesterday's protests in Tehran, state-run television said today.
The news came as Iran braced itself for the possibility of further post-election confrontations. Unconfirmed reports suggested the death toll could be much higher.
State television said the deaths happened during clashes between police and "terrorist groups".
Reuters reported that two petrol stations were set on fire, citing Iran's English-language Press TV, which described the culprits as "rioters".
Meanwhile, key figures in the Iranian regime have hit out at western responses to the disputed election in which the incumbent president, Mahmoud Ahmadinejad, was declared to have won a landslide victory.
The US president, Barack Obama, and the prime minister, Gordon Brown, have both stepped up their criticism of the violent crackdown in Iran, prompting Ahmadinejad to tell them to "stop interfering".
"Definitely by hasty remarks you will not be placed in the circle of friendship with the Iranian nation," he was quoted as saying in a meeting with clerics and scholars. "Therefore I advise you to correct your interfering stances." .
In yesterday's confrontations, thousands of protesters clashed with black-clad police wielding guns, truncheons, tear gas and water cannon.
Amateur video showed the demonstrators pelting police with stones and shouting: "Death to the dictator."
Scores of injured protesters who had sought medical treatment were arrested by security forces at hospitals in the capital, the International Campaign for Human Rights in Iran claimed.
The organisation said doctors had been ordered to report protest-related injuries to the authorities, and some seriously injured protesters had sought refuge at foreign embassies in an attempt to evade arrest.
Violence also spread to other cities, including Shiraz, where police were said to have lashed out at members of the public who were not involved in protests.
The demonstrations appeared to draw fewer people than recent mass rallies for Mir Hossein Mousavi, the opposition candidate who insists he beat Ahmadinejad in the 12 June elections.
The rallies left questions about Mousavi's ability to hold together his protest movement.
He bewildered many followers by not directly replying to the ultimatum issued by Ayatollah Ali Khamenei, Iran's most powerful figure, on Friday to call off the demonstrations or risk being held responsible for "bloodshed, violence and rioting".
Other footage posted in the hours after the crackdown showed blood pouring from a young woman's nose and mouth as frantic people tried to help her.
Two separate videos of the incident, each shot from a different angle, were uploaded on to Facebook and YouTube.
The YouTube video described the location of the incident as Amirabad, in central Tehran, and said the woman had been fatally shot.
Early today, Tehran's streets were quiet and there were no signs that security forces were massing.
But the chasm dividing the regime and the opposition widened further when it emerged that five female relatives of the former president, Hashemi Rafsanjani, had been arrested by security forces.
Rafsanjani is widely seen as being behind the opposition protest movement.
The five, including his daughter, Faezeh, were detained as they tried to join a protest in Tehran's Azadi Square yesterday.
The authorities are said to have offered to release four of them while keeping Faezeh, who was barred from leaving the country after taking part in a mass protest last week, but the four are understood to have refused to leave prison unless she is released.
The detentions suggested the mass demonstrations over Ahmadinejad's disputed re-election had escalated into an all-out power struggle between factions loyal to Khamenei and those loyal to Rafsanjani.
Rafsanjani, a pragmatist who favours detente with the west, is thought to have been orchestrating events behind the scenes in support of Mousavi.
Several sources suggested he had sought the backing of clerics on the experts' assembly – an important state body, which he heads – for a special session calling for Khamenei's removal.
Theoretically, the assembly's 86 members have the power to depose the supreme leader, but have never issued any public criticism of Khamenei.
The deep split between the two men, old comrades since before the 1979 Islamic revolution, was laid bare when Khamenei told a Friday prayer session at Tehran University his views were closer to Ahmadinejad's than those of Rafsanjani.
Further evidence of the polarisation within the regime was revealed by an uncompromising statement from Mousavi, who was said to have told supporters he was "ready for martyrdom".
On his website, he said the authorities would be responsible for any bloodshed resulting from a violent crackdown against protestors – a direct challenge to Khamenei's declaration on Friday that such responsibility would be borne by the protests' leaders.
"We are not against the Islamic system and its laws but against lies and deviations and just want to reform it," said the statement, posted after yesterday's violence.
"The people expect from their officials honesty and decency as many of our problems are because of lies … the Islamic revolution should be the way it was and the way it should be."
Even the regime's loyalists did not appear fully behind Khamenei, who has declared Ahmadinejad's re-election legitimate and called for an end to demonstrations.
The parliament speaker, Ali Larijani, told state television those questioning the result should be respected and not confused with a "group of bandits".
He also criticised the powerful guardian council – which has offered to recount a random 10% sample of the votes – for pro-Ahmadinejad bias.
"Though the members of the guardian council are religious, I wish some of them hadn't taken the side of one particular candidate," he said.
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