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 and saying it showed he was the winner.
"I will not submit to this dangerous charade," he said. He had announced a long list of alleged irregularities, including thousands of his poll monitors being barred from the 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 websites had been shut down.
Ayatollah Ali Khamenei, Iran's top political and religious leader, ignored the dispute over Friday's vote and hailed the 80% turnout as a great victory for the nation against the plots of its enemies.
"Your epic Friday 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 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 and effusively praised, demonstrators allied with Mousavi defied Iran's restrictions on unauthorized public gatherings and began 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 head scarf ripped off screamed defiantly at the stunned security officers who had just beaten her.
Riot police chased demonstrators down streets, beating and bloodying those who refused to move, and running off as the demonstrators fought back with rocks.
Along nearby Mirdamad Street, a major thoroughfare, shopkeepers urged panicked pedestrians into their stores for protection, in one instance locking the gate as a group of black-clad, truncheon-wielding riot police approached menacingly. Residents in nearby high-rises cheered on the protesters.
