TEHRAN (Reuters) - An Iranian opposition website said three pro-reform protesters were killed when police opened fire in Tehran on Sunday in a second day of violence during a Shi'ite Muslim religious mourning ritual.
The Jaras website, in reports that could not immediately be independently verified, later said another protester was killed in clashes between opposition supporters and security forces in the capital, without giving details.
"Protesters chanted anti-government slogans when carrying his body," Jaras said, adding fierce clashes also erupted in the central cities of Isfahan and Najafabad.
There was no immediate official comment on the reports.
Foreign media have been banned from reporting directly from opposition demonstrations after a disputed presidential election in June.
The reported unrest underlined escalating tension in the Islamic Republic six months after the election plunged the oil producer into turmoil and exposed splits within the clerical and political establishment.
"Three people were killed and two others were wounded when police opened fire at protesters," Jaras said about the situation in downtown Tehran.
Shooting was also heard elsewhere in a central area of the capital, it said.
The same website earlier said security forces fired tear gas to disperse opposition supporters in Tehran.
It said protesters set one police motorbike on fire. A plume of black smoke could be seen above the centre of the city, Jaras added, as police blocked streets in the area 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.
PRO-GOVERNMENT FORCES GATHER
Other witnesses said thousands of pro-government Iranians were also gathering in central Tehran.
A mobile phone network appeared to be down, Tehran residents said.
The authorities have warned the pro-reform opposition against using a two-day Shi'ite Muslim religious mourning ritual on Dec. 26-27 to revive protests against the clerical establishment.
The post-election unrest is the biggest in the Islamic Republic's 30-year history. Authorities deny opposition charges that voting was rigged to secure victory for President Mahmoud Ahmadinejad and have portrayed the protests as a foreign-backed bid to topple the Islamic establishment.
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 power in January.
(Editing by Angus MacSwan)