TEHRAN - Five people died in Tehran on Sunday when pro-reform protesters clashed with security forces, police said, in the worst outbreak of violence since June's contested presidential election sparked political turmoil.
Opposition websites said eight people were killed in Tehran and other cities across Iran when tens of thousands took to the streets during a religious festival.
Police in the capital said they were investigating the "suspicious" deaths, the IRNA news agency reported.
Among the dead was opposition leader Mirhossein Mousavi's nephew. State TV said "unknown assailants" killed Ali Habibi Mousavi Khamene, whose death was described as a "martyrdom" by a Mousavi ally.
It is the first time people have died in street protests since the immediate aftermath of the June presidential election
in which the opposition says more than 70 people were killed.
The authorities have estimated the post-vote death toll at about half that number, including pro-government militiamen.
Opposition website Jaras said police shot and killed four protesters in central Tehran. State TV dismissed foreign media reports that security forces had killed protesters. It said police had fired into the air to disperse demonstrators
Jaras said clashes continued in Tehran on Sunday night and police fired tear gas to disperse protesters in some squares.
"More protesters have gathered near Tajrish square in the past few minutes ... Police have increased their violence after more people gathered ... near the state-run radio and television building," Jaras said.
"Police have not been able to disperse protesters despite using tear gas," it said in a report that could not be independently verified. Foreign media have been banned from reporting directly from opposition protests since the June election.
A journalist for Dubai TV went missing on Sunday in Tehran, a source at the television station said.
More than 300 protesters were arrested in Tehran during the day, state TV said, adding that several banks, bus stops and trash cans were set on fire during the protests.
"Dozens of police officers have been injured including Tehran's police chief," Iran's deputy police chief Ahmadreza
Radan told state TV. "One person fell from a bridge, two died in car accidents and one was shot dead, but not by police."
On Sunday evening, opposition supporters in northern Tehran chanted Allahu Akbar (God is Greatest) from rooftops, as they did in the months after the election. Shots were heard in the area.
Jaras said unrest spread to other parts of Iran, including the holy city of Qom. Clashes erupted in the cities of Shiraz, Isfahan, Najafabad, Mashhad and Babol, it said, "We will kill those who killed our brothers," Jaras quoted demonstrators as chanting.
The White House condemned what it called the "unjust suppression" of civilians by the Iranian government and said the United States was on the side of protesters.
"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.
France condemned "the arbitrary arrests and violence committed against protesters".
Iranian 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.
"The Iranian nation has shown tolerance so far but they should know that the ... system's patience has a limit," Mojtaba Zolnour, a representative of Supreme Leader Ayatollah Ali Khamenei in the Revolutionary Guards, said, the semi-official Fars News Agency reported.
It said Mousavi's supporters "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.
Jaras website said at least four protesters were killed and many others wounded in the city of Tabriz, a Mousavi stronghold.
A witness in Tabriz told Reuters that people poured into the streets, chanting anti-government slogans.
Photographs from the clashes showed one man whose face was covered in blood, lying on the ground. Other pictures showed burning motorbikes and protesters throwing stones at police.
This year's Ashura coincided with the seventh day of mourning for leading dissident cleric Grand Ayatollah Hossein
Ali Montazeri, who died a week ago at the age of 87 in Qom.
A spiritual patron of Mousavi's movement, he was a fierce critic of the hardline clerical establishment.
Baton-wielding riot police also fired tear gas and warning shots to disperse Mousavi supporters on Saturday, Jaras reported.
Despite scores of arrests and security crackdowns, opposition protests have flared repeatedly since the poll, which
the opposition says was rigged to secure President Mahmoud Ahmadinejad's re-election. The authorities deny the claim.
The election turmoil has complicated the international dispute over Iran's nuclear programme, which the West believes may have military ends, not just civilian purposes.