Iran's police said on Sunday that four people were killed and 300 others arrested during clashes between anti-government protesters and security forces in Tehran, Iran's satellite channel Press TV reported.
"One died after falling from a bridge, two others were run over by cars and the fourth was shot dead by unknown assailants," Iran' s deputy police chief Ahmad Reza Radan was quoted as saying.
Iranian police were not involved in the killings and the incidents were under investigation, Radan said, adding that over 300 people were arrested during the protests, Xinhua reported.
An Iranian reformist website said earlier that the nephew of Iran's opposition leader and defeated presidential candidate Mirhossein Mousavi was killed Sunday during the clashes in Tehran.
"Ali Mousavi was shot near his heart today at noon and died later in a Tehran hospital," said Parlemannews, a website run by the Iranian parliament's minority reformist faction.
However, according to Press TV, police dismissed reports that Mousavi's nephew was among the dead, saying the man claimed to be Mousavi's nephew was not a relative of the defeated presidential candidate.
Sporadic clashes were seen in some parts of downtown Tehran and police forces fired shots into the air and tear gas to disperse protestors, Press TV reported.
According to witness accounts, several banks, bus stops and a number of trash-cans were set on fire during the protests.
Meanwhile, pro-government demonstrators also took to the streets to protest against the opposition, shouting slogans in support of the government, Press TV said.
The unrest took place as millions of Iranians held rallies on Sunday across the country to commemorate the Shiite Muslim ritual of Ashura.
State television footage showed large crowds of people gathered in every major Iranian cities to commemorate Ashura, which marks the death of Imam Hussein, grandson of Prophet Mohammed, who was killed and buried in Karbala in 680 AD.
The Ashura ritual is performed in Iran and some other countries with large populations of Shiite Muslims. During the annual Ashura commemorations, mourners, generally dressed in black, took to the streets or gathered in mosques to grieve over the death of Hussein.