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.

Gunfire was also heard in three districts of wealthy northern Tehran, residents said.

Speaking before the shooting, Mousavi said he was ready to pay any price in his fight against election irregularities, his Web site quoted him as saying.

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.

The Basij militia is a volunteer paramilitary force fiercely loyal to Supreme Leader Ayatollah Ali Khamenei who has the final say on all matters of state in Iran.

Earlier during the rally, a Reuters reporter said Mousavi supporters had formed a human chain outside the Basij building in order to prevent any trouble when demonstrators passed it.

Shouting Allahu Akbar (God is greatest), the crowds 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.

Mousavi said he was not optimistic about his appeal against the result of Friday's election to the 12-man Guardian Council, six of them selected by Khamenei, six elected by parliament.

Many of its members during the election were not impartial and supported the government candidate, Mousavi said, referring to Ahmadinejad. I'm urging government forces to stop violence against people, Mousavi said.

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.

Residents said there had also been peaceful pro-Mousavi demonstrations in the cities of Rasht, Orumiyeh, Zahedan, and Tabriz on Monday.

MILES OF PROTESTORS

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 cellphones -- even though his words could not be heard above the noise of the crowd.

Mousavi, smiling and looking relaxed, had said he was ready in case the election was re-run, state television said.

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

U.S. President Barack Obama has said he is ready to engage with Iran if it unclenches its fist, but 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 Ian Kelly, adding that Washington was still assessing what had happened in the election.

The European Union increased pressure on Iran to agree to opposition demands to investigate Ahmadinejad's landslide election victory and halt a crackdown on protesters.

France, Germany and Britain led the EU campaign to persuade Iran to clarify the election results.

In Paris, Foreign Ministry spokesman Eric Chevallier said Iran's ambassador had been summoned to hear French concerns over the brutal repression of peaceful protests and the repeated attacks on the liberty of the press and freedom of speech.

Britain said it was worried that events in Iran might affect any future international engagement with its government.

The implications are not yet clear, said British Foreign Secretary David Miliband. What we know is that there has been no Iranian response to the outreach that has been made by the international community, including the United States.

The protests over Ahmadinejad's re-election are the sharpest display of discontent in the Islamic Republic for a decade -- and have drawn broader support than the student unrest of 1999.

I just want to show the president that we are not bandits, said Maryam Sedaghati, a pro-Mousavi demonstrator in her 20s wearing a green headscarf. I want my vote back.

A retired 61-year-old teacher who gave his name only as Ali said the rally recalled the 1979 Islamic revolution. We used to protest against the shah in this street. I'm so sorry that now we have to walk the same street to preserve our rights.

(Editing by Jon Hemming and Jon Boyle)

United kingdom