Iran protesters killed, including Mousavi's nephew
Government forces' motorbikes on fire in central Tehran
At least four people have been killed and hundreds arrested amid violence between anti-government protesters and police in the Iranian capital, Tehran.
Opposition sources said the nephew of reformist leader Mir Hossein Mousavi was among those killed when police opened fire.
Officials say three people died in accidents and one was shot dead but not by police, who they say were unarmed.
Witnesses say clashes are continuing overnight in parts of the city.
Opposition website Jaras said police were firing tear gas to disperse protesters, but more demonstrators had gathered near the state-run radio and television headquarters.
Other opposition websites have also reported four deaths in Tabriz, north-western Iran, however there is no confirmation of this.
Jon Leyne, BBC News Tehran correspondent
The opposition hoped for a massive day of demonstrations, and they have managed that beyond their expectations.
Despite attempts by the security forces to disperse them, the protesters eventually took over a large section of central Tehran, leaving the police watching from the sidelines. And there are similar reports from across the country.
For much of the morning there was a series of violent confrontations.
Witnesses described how opposition supporters attacked the police with their bare hands, and the police eventually opened fire directly on the crowd.
The size of the demonstrations, and the death of a number of protesters, could dramatically change the nature and the intensity of the confrontation.
But neither side has a clear strategy of what to do next. The opposition is leaderless. The government is still pretending there are just a handful of troublemakers.
From day to day, it is not clear how the crisis will develop.
Leaders of opposition parties had urged their supporters to take to the streets on Sunday as the Shia Muslim festival of Ashura reached a climax.
People were chanting "Khamenei will be toppled", opposition sources said, a reference to Iran's Supreme Leader.
According to witnesses, demonstrators defied attempts by security forces to disperse them and some attacked police.
Police responded by firing into the crowd, opposition sources say, although this is denied by the authorities.
"Police [have] not fired any shots in Tehran and security forces were not in possession of firearms," Tehran's police chief Commander Azizollah Rajabzadeh was quoted as saying by the Iranian ISNA news agency.
Commander Rajabzadeh was among dozens of security force members injured in the clashes, Iranian state-owned channel Press TV reported.
Iran's deputy police chief, Ahmad-Reza Radan, said one of those killed fell from a bridge, two others were hit by cars and one other was shot dead "by an unknown assailant".
He said the fatal shooting was being investigated.
Mr Mousavi's website said his nephew, Seyed Ali Mousavi, was shot in the back as security forces fired on demonstrators
Mr Mousavi rushed to a Tehran hospital where his nephew had been taken, the website added.
As the clashes escalated, police helicopters were seen flying over central Tehran and clouds of smoke billowed into the sky, reports said.
Security forces clashed with protesters trying to reach central Enghelab Square, witnesses said.
Protesters chanted "this is the month of blood" and called for the downfall of Ayatollah Khamenei, according to opposition websites.
At the same time, pro-government demonstrators marched on Enghelab Street in support of the ayatollah, witnesses said.
Protests were also reported in the cities of Isfahan and nearby Najafabad.
In a statement, the White House said it strongly condemned the "unjust suppression of civilians in Iran".
The French foreign ministry said it condemned the "arbitrary arrests and the violent actions" committed against protesters defending "their right to freedom of expression and their desire for democracy".
The French government has continued to lobby the Iranian authorities to release a French university lecturer who was charged with spying during the election. Clotilde Reiss remains in Tehran, and last appeared in court on 23 December.
Tensions have risen in Iran since influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri died a week ago aged 87.
Mr Mousavi' supporters have sought to use Shia religious festivals to show continued defiance of President Mahmoud Ahmadinejad's government.
Denied the right to protest, the opposition chose the highly significant festival of Ashura when millions of Iranians traditionally take to the streets for ceremonies and parades, BBC Tehran correspondent Jon Leyne says.
The festival mourns the 7th Century death of Imam Hussein, a grandson of the Prophet Muhammad.
Mr Mousavi came second in the June election, and anger at the result saw mass protests in Tehran and other cities that led to thousands of arrests and some deaths.
Mr Mousavi has said the poll, that returned Mr Ahmadinejad to power, was fraudulent.
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