Tens of thousands of supporters of Iran's defeated presidential candidate Mirhossein Mousavi gathered in downtown Tehran on Monday, defying an Interior Ministry ban.

Shouting Allahu Akbar (God is great), they converged on Revolution Square, where the moderate former prime minister was expected to call for calm after two days of violent unrest in the capital since hardline President Mahmoud Ahmadinejad was declared the victor in Friday's vote.

Mousavi, take back our votes, the marchers chanted as they waited for Mousavi and other pro-reform leaders who back his call for the election result to be overturned.

Stick-wielding men on motorcycles scuffled with some of the marchers, who wore Mousavi's green campaign colors.

The election outcome has disconcerted Western powers trying to induce the world's fifth-biggest oil exporter to curb nuclear work they suspect is for bomb-making, a charge Iran denies.

The European Union plans to demand clarification of Ahmadinejad's victory and voice concern at the treatment of his opponents, Spanish Foreign Minister Miguel Angel Moratinos said after talks in Luxembourg with EU counterparts.

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.

German Chancellor Angela Merkel said her country was very worried about the aftermath of Iran's election, which she said had been marked by signs of irregularities.

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.

U.S. leaders have reacted cautiously, in the hope of keeping alive President Barack Obama's strategy of engagement with Iran.

The protests have marked the sharpest display of discontent in the Islamic Republic in years.

Mousavi has asked the watchdog Guardian Council to annul the result, citing irregularities. The Interior Ministry and the president have rejected charges of fraud.

Supreme Leader Ayatollah Ali Khamenei, who has upheld the election result, met Mousavi on Sunday and told him to pursue his complaints calmly and legally, state television said.

KHATAMI SPEAKS OUT FOR MOUSAVI

Iran's reformist former president, Mohammad Khatami, assailed the authorities for denying permission for the pro-Mousavi rally and said the election had dented public trust.

The 12-man Guardian Council, whose chairman, Ayatollah Ahmad Jannati, endorsed Ahmadinejad before the vote, said it would rule within 10 days on two official complaints it had received from Mousavi and another losing candidate, Mohsen Rezaie.

The council vets election candidates and must formally approve results for the outcome to stand.

Ebrahim Yazdi, leader of the banned opposition Freedom Movement, said Ahmadinejad's attacks on his opponents had opened a Pandora's box which had led to a deep crisis within the Islamic state's establishment.

The result of such a crisis now is that the rift among the ... personalities in the revolution is getting deeper, he said. It is also between people and their government ... a rift between state and the nation. It is the biggest crisis since the (1979) revolution.

Ahmadinejad delayed a visit to Russia on Monday for a regional summit, but would arrive on Tuesday, an Iranian embassy source in Moscow said, giving no reason for the change of plan.

About 400 pro-reform students, many wearing green face masks to conceal their identity, gathered earlier at a mosque in Tehran University and demanded Ahmadinejad's resignation.

Some said religious militia had attacked their dormitory. They hit our friends and took away at least 100 students. We have no news about their whereabouts, said another student.

University officials denied the reported incidents.

A burned-out shell of a bus or truck lay inside the university's dormitory compound, Reuters television pictures showed. Across the road, a bank office had been gutted by fire.

Outside the British embassy in downtown Tehran, Ahmadinejad supporters chanted slogans against the plots of Iran's Western enemies, a Reuters witness said.

Israeli Defense Minister Ehud Barak again said his country reserved all options on Iran -- code for a possible attack on nuclear sites, which Tehran says are only for peaceful purposes.

The triumph of extremism is bad news, as any victory of extremists should be defined, he told reporters at the Paris Air Show, referring to the outcome of the Iranian election.

I am not sure to what extent the result reflects the real will of the Iranian people, added Barak, whose country is believed to have the Middle East's only nuclear arsenal.

(Editing by Alistair Lyon and Janet Lawrence)

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