Images from Tehran showed police motorcycles being set alight
Clashes have taken place overnight in the Iranian capital, Tehran, following anti-government protests on Sunday in which at least eight people died.
Reports say police fired tear gas to disperse crowds of demonstrators in various parts of the city overnight.
The nephew of opposition leader Mir Hossein Mousavi was among those killed in Sunday's clashes, the worst violence since June's contested elections.
His funeral on Monday is thought likely to be a focal point for more protests.
According to Mr Mousavi's website, Seyed Ali Mousavi was shot in the back as security forces fired on demonstrators.
On Monday, state-owned English-language Press TV said eight people had died. Earlier, Persian state television had reported at least 15 people killed.
Police denied being responsible for any of the deaths. They said three were accidents and that one person had been shot, but not by police. Details of the others have not been given.
Officials said the shooting was under investigation.
Opposition politician Ebrahim Yazdi, a foreign minister after the 1979 revolution and now leader of the Freedom Movement of Iran, was also arrested in the early hours of Monday.
His son Khalil, who lives in the US, told the BBC's World Today programme he believed the Iranian authorities wanted to close down all opposition groups.
"It is a shameless and irresponsible act. [They are] arresting a 78-year-old man who has stood for nothing but freedom and open society within Iran all of his life," he said.
"Any opposition now, they want to shut [it] down. We're going down a one-way street that's now going downhill."
The protests, which began after President Mahmoud Ahmadinejad's controversial re-election in June, have grown into the biggest challenge to the government since the 1979 Islamic revolution.
The official death toll for Sunday's confrontation is the highest since June.
About 300 people were detained after Sunday's protests, police said.
Those arrested include members of the banned opposition group Mujahideen Khalq Organisation (MKO) - or the People's Mujahideen - Press TV reported.
The report quoted a source within the Iranian intelligence ministry.
Opposition supporters took to the streets on Sunday as the Shia Muslim festival of Ashura reached its climax.
Tehran's police chief, Azizollah Rajabzadeh, was among dozens of security force members injured in the clashes, officials said.
Unconfirmed reports said four people also died in protests in the north-western city of Tabriz and there were clashes in Isfahan and Najafabad in central Iran and Shiraz in the south.
Foreign media face severe restrictions in Iran and reports cannot be verified. However, witnesses said that in Tehran some protesters attacked police.
Police responded by firing directly into the crowds, opposition sources say, although this is denied by Iranian authorities.
Clashes continued throughout the day. In the early hours of Monday, opposition sources said a large crowd had also gathered near the offices of the state-run television and radio.
They said police were firing tear gas in an attempt to disperse them.
Moderate cleric Mehdi Karoubi, who came fourth in last June's election, criticised Iran's rulers for Sunday's violence, an opposition website reported.
"What has happened to this religious system that it orders the killing of innocent people during the holy day of Ashura?" the Jaras website reported him as saying.
"Why is such a holy day not respected by the rulers?"
The White House condemned the "unjust suppression" of protests.
RECENT UNREST IN IRAN
19 Dec: Influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri dies aged 87
21 Dec: Tens of thousands attend his funeral in Qom; reports of clashes between opposition supporters and security forces
22 Dec: Further confrontations reported in Qom
23 Dec: More clashes reported in city of Isfahan as memorial is held
24 Dec: Iran reportedly bans further memorial services for Montazeri except in his birthplace and Qom
26 Dec: Clashes reported in central and northern Tehran
27 Dec: At least eight dead following anti-government protests in Tehran; 300 reported arrested
"Hope and history are on the side of those who peacefully seek their universal rights, and so is the United States," White House National Security Council spokesman Mike Hammer said.
A spokesman for the French foreign ministry, Bernard Bolero, also condemned the violence.
"These people in the streets are just claiming more freedom, more democracy," he told the BBC. "The repression of the police forces is not acceptable."
Iranian security forces have been on alert since influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri died a week ago aged 87.
His funeral attracted tens of thousands of pro-reform supporters, many of whom shouted anti-government slogans.
BBC Tehran correspondent Jon Leyne, who is now based in London, says the opposition - denied the right to protest - chose the highly significant festival of Ashura when millions of Iranians traditionally take to the streets for ceremonies and parades.
Anger at last June's elections, won by incumbent President Mahmoud Ahmadinejad, sparked mass protests in Tehran and other cities that led to thousands of arrests and some deaths.
Mr Mousavi and other opposition leaders have said the poll was rigged.
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