Iran's supreme leader ordered Monday an investigation into allegations of election fraud, marking a stunning turnaround by the country's most powerful figure and offering hope to opposition forces who have waged street clashes to protest the re-election of President Mahmoud Ahmadinejad. State television quoted Ayatollah Ali Khamenei directing a high-level clerical panel, the Guardian Council, to look into charges by pro-reform candidate Mir Hossein Mousavi, who has said he is the rightful winner of Friday's presidential election. “Issues must be pursued through a legal channel,” state TV quoted Khamenei as saying. The supreme leader said he has “insisted that the Guardian Council carefully probe this letter.” Iran's 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. Mousavi appeared in public Monday for the first time since the election, joining thousands of supporters who defied a ban to stage a mass rally in Tehran. Addressing the crowds from atop a car, he said he was ready to take part in a new election. “The vote of the people is more important than Mousavi or any other person,” the former prime minister told the massive rally. Stick-wielding men on motorcycles scuffled with some of the marchers, who wore Mousavi's green campaign colors. “Between 1.5 million to two million people have gathered,” one policeman involved in the security operation for the protest told a news agency. The election outcome has disconcerted Western powers. 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. UN chief Ban Ki-moon said that the “genuine” will of the Iranian people as expressed in the disputed presidential elections should be “fully respected.” “The genuine will of the Iranian people should be fully respected,” Ban said in answer to a question about allegations of vote-rigging in the polls that returned President Mahmoud Ahmadinejad to power. 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. US leaders have reacted cautiously, in the hope of keeping alive President Barack Obama's strategy of engagement with Iran. British Foreign Secretary David Miliband made clear doubts about the fairness of the election could have an impact on talks on Iran's nuclear program.