Iran's hardline Islamic Basij militiamen killed at least one person on Monday and wounded more when their building was attacked by demonstrators protesting an election they say was stolen by President Mahmoud Ahmadinejad.
An Iranian photographer at the scene witnessed the shooting, which came during a demonstration by tens of thousands in the capital Tehran in support of opposition candidate Mirhossein Mousavi who has appealed the election result.
Shooting was heard in three districts of northern Tehran, residents said.
Members of Iran's security forces have at times fired into the air during two days of the Iranian capital's most violent unrest since the 1979 Islamic Revolution, and used batons to beat protesters who have pelted police with stones.
Shouting Allahu Akbar (God is greatest), the crowds earlier converged on Revolution Square, where Mousavi addressed part of the crowd through a loud hailer and held his fists clenched above his head, in a sign of victory.
The protest took place in defiance of an Interior Ministry ban and was a reply to Ahmadinejad's state-organized victory rally, which also drew vast crowds to Azadi Square on Sunday.
Supporters stretching along several kilometers (miles) of a Tehran boulevard waved green flags, Mousavi's campaign colors, and held portraits of him aloft as they tried to take pictures on their cell phones -- even though his words could not be heard above the noise of the crowd.
Iran's state television said Mousavi, smiling and looking relaxed in a striped shirt, had said he was ready in case the election was re-run.
Mousavi, take back our votes, the marchers chanted before Mousavi appeared, along with other pro-reform leaders who backed his call for Friday's election result to be overturned.
Ebrahim Yazdi, leader of the banned opposition Freedom Movement, said Ahmadinejad's attacks on his opponents had opened a Pandora's box of divisions within the establishment and between the people and their government.
It is the biggest crisis since the revolution, he said.
The disputed election has dismayed Western powers trying to induce the world's fifth-biggest oil exporter to curb nuclear work that they suspect is for bomb-making, a charge Iran denies.
U.S. DEEPLY TROUBLED
The U.S. State Department said it was deeply troubled by reports of violence and voting irregularities in the election.
We are deeply troubled by the reports of violence, arrests and possible voting irregularities, said State Department spokesman Iran Kelly, adding that Washington was still assessing what had happened in the election.