An Iranian protester flashes the peace, or victory, sign during anti-government protest at the Enqelab (Revolution) Street in Tehran on Sunday. ((Associated Press))

Security forces opened fire on anti-government protesters in the Iranian capital Sunday, killing at least five people, according to opposition websites and witnesses.

The dead included a nephew of opposition leader Mir Hossein Mousavi, according to Mousavi's website.

Authorities deny reports that some protesters were shot dead. Police, who denied using firearms, said dozens of officers were injured and more than 300 protesters were arrested.

"So far there have been no reports of killings and no one has been killed up to now," Tehran police Chief Azizollah Rajabzadeh told the Iranian Students' News Agency.

Witnesses said another victim was an elderly man who had a gunshot wound to the forehead. He was seen being carried away by opposition supporters.

There were unconfirmed reports that four people died in protests in Tabriz in northwest Iran, the pro-reform Rah-e-Sabz website said. Fierce clashes also broke out in Isfahan and Najafabad in central Iran and Shiraz in the south, the website said.

A video posted to YouTube shows protesters setting garbage bins on fire and facing off with police in Tehran.

Journalists are banned from covering opposition protests in the country and there is no way to independently confirm what happened.

Thousands of opposition supporters were reported to be on the streets, chanting "Death to the dictator," a reference to hardline President Mahmoud Ahmadinejad, and protesting the disputed June election, which supporters say reformist candidates won.

Reuters News Agency reported that police were using tear gas to clear the streets.

The demonstration coincided with observance of the holy day Ashoura, commemorating the seventh-century death in battle of one of Shia Islam's most beloved saints, Imam Hussein — a time when many Iranians take part in traditional street parades.

With files from The Associated Press