TEHRAN - Supporters of Iran's defeated presidential candidate plan a rally in Tehran on Monday to protest against the re-election of Mahmoud Ahmadinejad, which has sparked two days of violent demonstrations in the capital.

Former Prime Minister Mirhossein Mousavi has appealed to the Islamic Republic's top legislative body to annul Friday's election result, in which hard-liner Ahmadinejad took 63 percent of the vote, because of what he alleges were irregularities.

Ahmadinejad himself held a triumphant rally on Sunday, attended by a cheering crowd of tens of thousands of people. It was not clear whether authorities would allow any demonstration by his opponents.

The unrest that has rocked Tehran and other cities since results were declared on Saturday is the sharpest show of discontent against the Islamic Republic's leadership for years.

The election result has disconcerted Western powers trying to induce the world's fifth-biggest oil exporter to curb its nuclear program. U.S. President Barack Obama had urged Iran's leadership to unclench its fist for a new start in ties.

Pro-Mousavi demonstrators threw stones at police at Tehran University on Sunday and also clashed with Ahmadinejad supporters on a main street in the city that was littered with broken glass and fires.

In the north of the capital, a stronghold of Mousavi backers, riot police patrolled streets after midnight. Rubbish burned in the street, some cars had their windows broken, and police blocked access to roads.

MOUSAVI'S APPEAL

In a statement on his website, Mousavi said he had formally asked Iran's legislative Guardian Council to cancel the election result.

I urge you, Iranian nation, to continue your nationwide protests in a peaceful and legal way, he said.

Mousavi's supporters handed out leaflets calling for a rally in Tehran on Monday afternoon. After dusk some took to rooftops across the city calling out Allah Akbar (God is greatest), an echo of tactics by protesters in the 1979 Islamic revolution.

Ahmadinejad appeared at his rally amid a sea of red, white and green Iranian flags waved by supporters thronging Tehran's Vali-e Asr square, some perched on rooftops or cars, to applaud his win.

Some ... say the vote is disrupted, there has been a fraud. Where are the irregularities in the election? Ahmadinejad said in a speech that the crowd punctuated with roars of approval.

Some people want democracy only for their own sake. Some want elections, freedom, a sound election. They recognize it only as long as the result favors them, he said.

U.S. Vice President Joe Biden cast doubt on the election result but said Washington was reserving its position for now.

It sure looks like the way they're suppressing speech, the way they're suppressing crowds, the way in which people are being treated, that there's some real doubt, he told NBC's Meet the Press when asked if Ahmadinejad had won the vote.

Germany, one of Iran's biggest trading partners and a negotiator in the West's nuclear talks with Tehran, said it had summoned the Iranian ambassador.

We are looking toward Tehran with great concern at the moment. There are a lot of reports about electoral fraud, Foreign Minister Frank-Walter Steinmeier told German ZDF TV.

An adviser to French President Nicolas Sarkozy said what was happening in Iran was clearly not good news for anyone, neither for the Iranians nor for peace and stability in the world.

Iran's refusal to halt atomic work the West suspects is aimed at making bombs, a charge Tehran denies, has sparked talk of possible U.S. or Israeli strikes on its nuclear sites.

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