An Iranian opposition website said four people were killed in clashes between pro-reform protesters and security forces in Tehran today but the capital's police chief denied the report.
Violence erupted in Tehran for a second day during a Shi'ite Muslim religious mourning ritual and unrest also spread to other parts of Iran, including the holy Shi'ite city of Qom, the opposition Jaras website said.
It underlined escalating tension in the Islamic Republic six months after a disputed presidential poll plunged the oil producer into turmoil and exposed widening splits within the clerical and political establishment.
Jaras, in reports that could not be independently verified, said police shot dead three protesters in central Tehran. It later said a fourth demonstrator was killed in clashes in the capital, without giving details.
It was the first report of killings in street protests since widespread unrest and violence in the immediate aftermath of the June poll in which the opposition says more than 70 people died.
Among those who are reported to have died in the clashes was a nephew of Iranian opposition leader Mirhossein Mousavi.
The authorities have estimated the post-vote death toll at about half that number, including pro-government militiamen.
Tehran police chief Azizollah Rajabzadeh, speaking about today's protests, said "So far there have been no reports of killings and no one has been killed up to now," according to the ISNA news agency.
Tens of thousands of demonstrators had packed the streets of Tehran and clashes also erupted in the cities of Shiraz, Isfahan, Najafabad, Mashhad and Babol, Jaras said.
It said anti-government demonstrations would continue in various Tehran squares this evening.
"The opposition has decided to hold protests in Vanak, Mohseni, Enqelab and Tajrish squares in Tehran as well as in main public parks," it said.
English-language state television reported sporadic clashes in Tehran and said a bank and bus stop were set on fire.
The semi-official Fars News Agency said supporters of opposition leader Mirhossein Mousavi and pro-reform cleric Mehdi Karoubi "followed the call of the foreign media" and took to the streets -- a reference to the government position that the unrest is being stoked by foreign enemies of the Islamic Republic.
It said the group of "deceived hooligans" damaged public and private property and "disrespected" the holy Shi'ite day of Ashura, without elaborating.
Foreign media have been banned from reporting directly from opposition demonstrations since the June election.
Jaras said protesters set one police motorbike on fire. A plume of black smoke could be seen above the city centre as police blocked streets and clashes intensified.
Jaras later said police were refusing orders to shoot at protesters.
"Some of them try to shoot into the air when pressured by their commanders," it said.
A witness told reporters there was a heavy presence of both security forces and opposition supporters in central Tehran, a city of around 12 million people.
"Police prevented groups of protesters from joining each other," she said.
Other witnesses said thousands of pro-government Iranians had gathered in central Tehran.
Despite scores of arrests and security crackdowns, opposition protests have flared repeatedly since the June poll, which the opposition says was rigged to secure hardline president Mahmoud Ahmadinejad's re-election.
Reformist websites said there had also been clashes in Tehran yesterday, with baton-wielding riot police firing tear gas and warning shots to disperse Mousavi supporters.
The authorities had warned the opposition against using the two-day Shi'ite Muslim Tasoua and Ashura festival on December 26th and 27th to revive protests against the clerical establishment.
This year's Ashura coincided with the traditional seventh day of mourning for leading dissident cleric Grand Ayatollah Hossein Ali Montazeri, who died a week ago at the age of 87 in the holy Shi'ite city of Qom.
A spiritual patron of the movement of opposition leader Mirhossein Mousavi, he was a fierce critic of the hardline clerical establishment.
The unrest that erupted after the June vote is the biggest in the Islamic state's 30-year history. Authorities deny opposition charges that voting was rigged.
The unrest has complicated the international dispute over Iran's nuclear programme, which the West believes may have military ends, not just civilian purposes. World powers have set an end-of-year deadline for Iran to agree a U.N.-drafted deal to ship most of its low-enriched uranium abroad in exchange for fuel for a Tehran research reactor.
The turmoil has also set back tentative US moves towards a rapprochement with Iran initiated by president Barack Obama when he took office in January.
Ashura is one of the main Shi'ite holy days, when the faithful commemorate the slaying of the Prophet Mohammad's grandson Hussein in Kerbala in present-day Iraq in 680 AD.
State television showed live footage of big crowds taking part in the nationwide rituals.
Reuters