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Opponents of Mahmoud Ahmadinejad clashed with police in the heart of Iran's capital Saturday, pelting them with rocks and setting fires in the worst unrest in Tehran in a decade. They accused the hard-line president of using fraud to steal election victory from his reformist rival.

The brazen and angry confrontations — including stunning scenes of masked rioters tangling with black-clad police — pushed the self-styled reformist movement closer to a possible moment of truth: Whether to continue defying Iran's powerful security forces or, as they often have before, retreat into quiet dismay and frustration over losing more ground to the Islamic establishment.

But for at least one day, the tone and tactics were more combative than at any time since authorities put down student-led protests in 1999. Young men hurled stones and bottles at anti-riot units and mocked Ahmadinejad as an illegitimate leader. The reformists' new hero, Mir Hossein Mousavi, declared himself the true winner of Friday's presidential race and urged backers to resist a government based on "lies and dictatorship."

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Authorities, too, pushed back with ominous measures apparently seeking to undercut liberal voices: jamming text messages, blocking pro-Mousavi Web sites and Facebook and cutting off mobile phones in Tehran.

The extent of possible casualties and detentions was not immediately clear. Police stormed the headquarters of Iran's largest reformist party, the Islamic Iran Participation Front, and arrested several top reformist leaders, said political activists close to the party.The activists spoke on condition of anonymity because of the sensitivity of the situation.

Mousavi did not appear in public, but warned in a Web message: "People won't respect those who take power through fraud."

Many backers took this call to the streets. Thousands of protesters — mostly young men — roamed through Tehran looking for a fight with police and setting trash bins and tires ablaze. Pillars of black smoke rose among the mustard-colored apartment blocks and office buildings in central Tehran. In one side road, an empty bus was engulfed in flames.

Police fought back with clubs, including mobile squads on motorcycles swinging truncheons.

The scuffles began when protesters gathered hours outside the Interior Ministry around the time officials announced the final election results showing a nearly 2-to-1 landslide for Ahmadinejad. Demonstrators chanted "the government lied" and waved the ribbons of Mousavi's "green" movement — the signature color of his youth-driven campaign.