After security forces prevented journalists from attending an early afternoon news conference he tried to hold, Mousavi, a former prime minister, released a statement alleging a conspiracy to manipulate the vote results, which he claimed showed he was the winner.
"I will not submit to this dangerous charade," he said. He had announced a list of alleged irregularities, including thousands of his poll monitors being barred from voting stations, the previous night. Iran allows no independent observers to monitor the vote.
As the day drew to a close, both campaigns reported that the candidates were under house arrest hours after their offices and affiliated Web sites had been shuttered.
Ayatollah Ali Khamenei, Iran's top political and religious leader, ignored the dispute over Friday's vote and hailed the 80 percent turnout as a great victory for the nation against the plots of its enemies.
"Your epic [victory] was a striking and unprecedented event, in which the political growth, determined political visage and the civic capability and potential of the Iranian nation were beautifully and splendidly displayed before the eyes of the world," he said in a speech broadcast on state television.
The supreme leader's approval means Mousavi supporters have no legal recourse to contest the vote.
But even as the results were released, demonstrators allied with Mousavi defied Iran's restrictions on unauthorized public gatherings and began setting fire to garbage and assembling in rowdy protests.
A demonstration formed in Tehran's Vanak Square in midafternoon. Mostly young protesters, some wearing surgical masks to guard against tear-gas attacks, set fires and blocked traffic as older Iranians stood along the sidelines cheering them on, occasionally joining in the chanting.
Passing drivers honked in support. A woman with her headscarf ripped off screamed defiantly at the stunned security officers who had just beaten her.
Riot police chased demonstrators and some passers-by down streets, beating and bloodying those who refused to move, and running off as the demonstrators fought back with rocks.
Along Mirdadmad Street, a major thoroughfare, shopkeepers urged panicked pedestrians and protesters into their stores for protection, in one instance locking the gate as a group of truncheon-wielding riot police approached. Residents in nearby high-rise towers cheered on the protesters.
