Iranian reformist websites said security forces killed a nephew of opposition leader Mir Hossein Mousavi yesterday as police confirmed that four people died during anti-government rallies in Tehran.

Witnesses said dozens of protesters were also wounded in what turned into the bloodiest showdown between protesters and security forces since unrest in June following a disputed presidential election.

Opposition website Rahesabz.net said four protesters were shot dead by security forces in central Tehran while reformist website Parlemannews said Mr Mousavi's nephew died in hospital after being shot in the chest.

Police arrested more than 300 people, deputy police chief Ahmad Reza Radan told state television.

He confirmed four people had died but said one had fallen off a bridge and two had died in car accidents. The fourth had died of bullet wounds, but he denied that the security forces were responsible.

"As the police were not using firearms this is suspicious and it is being investigated," he said.

Police had earlier denied that anyone had died in the clashes, which witnesses said came after tens of thousands of opposition supporters took to the streets for a second straight day to use the Ashura rituals to protest.

The authorities' response again drew international criticism.

The White House strongly condemned "violent and unjust suppression" of civilians.

The strongly worded statement contrasted with careful initial responses by Washington after election protests in June and came as the showdown with Iran over its controversial nuclear programme reached a critical point.

The French Foreign Ministry condemned the police action against "simple protesters" and called for a political solution.

Witnesses said police used batons and tear gas in the crackdown, which followed stern warnings by the authorities that they would crush attempts to use Ashura processions as a launchpad for protests.

When this failed to disperse the crowds, they opened fire, websites said.

Parlemannews said Seyed Ali Mousavi, the 35-year-old nephew of Mr Mousavi, was shot near his heart during clashes at Enghelab square "and was martyred after he was taken to Ebnesina hospital".

Mr Mousavi and other family members were at the hospital along with some political figures, said the website, which is run by the Parliament's minority reformist faction which backs the opposition.

Opposition website Rahesabz.net reported four people killed but did not identify them.

"Three of our compatriots were martyred and two were injured in clashes. The (website) reporter who was on the scene said these three were directly shot at by military forces."

A fourth protester was killed later near Vali Asr intersection on Enghelab, it said.

AFP was not able to verify the various reports as the foreign media is banned from covering opposition demonstrations.

Syrian journalist, Reza al-Basha, 27, an Iran-based correspondent for Dubai TV, was feared to be among those arrested, a colleague said.