TEHRAN (Reuters) - Four people were killed in clashes between pro-reform protesters and security forces in Tehran on Sunday in a second day of violence during a Shi'ite Muslim religious mourning ritual, an opposition website said.
The casualties were the first reported killings in protests since the immediate aftermath of a disputed election in June in which the opposition says more than 70 people died.
The authorities have estimated the post-vote death toll at about half that number, including pro-government militiamen.
Sunday's violence underlined escalating tension in the Islamic Republic six months after the presidential poll plunged the oil producer into turmoil and exposed widening splits within the clerical and political establishment.
The Jaras website, in reports that could not immediately be independently verified, said police shot dead three pro-reform protesters in downtown Tehran. It later said another demonstrator was killed in clashes between opposition supporters and security forces in the capital, without giving details on how he died.
"Three people were killed and two others were wounded when police opened fire at protesters," it said.
The website later reported that police forces refused orders to shoot at protesters.
"Police forces are refusing their commanders' orders to shoot at demonstrators in central Tehran ... some of them try to shoot into air when pressured by their commanders," Jaras said.
Shooting was also heard elsewhere in the city centre and security forces had fired tear gas to disperse opposition supporters, it said.
Tens of thousands of demonstrators had packed the streets of Tehran and clashes also erupted in Shiraz, Isfahan, Najafabad and Babol cities, Jaras said.
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 -- reference to the government position that the unrest is being stoked by foreign governments opposed to 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 after the 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.
A witness told Reuters there was a heavy presence of both security forces and opposition backers 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 were also gathering in central Tehran.
MOURNING FOR DISSIDENT CLERIC
Despite scores of arrests and security crackdowns, opposition protests have repeatedly flared since the June poll, which the opposition says was rigged to secure hardline President Mahmoud Ahmadinejad's re-election.
Reformist websites reported clashes in Tehran also on Saturday, saying baton-wielding riot police fired tear gas and warning shots to disperse Mousavi supporters in various areas.
The authorities had warned the opposition against using the two-day Shi'ite Muslim Tasoua and Ashura festival on Dec. 26-27 to revive protests against the clerical establishment.
This year's Ashura on Sunday 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 for opposition leader Mirhossein Mousavi, he was a fierce critic of the hardline clerical establishment.
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.
It has complicated a long-running international dispute over its 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 efforts towards a rapprochement between Iran and its longtime foe the United States instigated by U.S. 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 but did not mention any clashes.
A Reuters photographer said police took his press accreditation on Sunday for taking pictures of Ashura mourning rituals outside Tehran.
"Police deleted all the pictures I had with any sign of green," the photographer said, referring to the symbolic colour of the opposition movement.
(Editing by Angus MacSwan)