Tens of thousands of supporters of Iran's defeated presidential candidate Mirhossein Mousavi gathered in downtown Tehran today, 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."

US 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.

• Ahmadinejad wins Iran vote, more clashes erupt