A text message sent to the cellphones of some people called on them to join in the march today, expected to be attended by Mousavi, reformist former President Mohammad Khatami and Mehdi Karroubi, another presidential candidate.
Analysts were not surprised by Khamenei's tone, which defined further civil disobedience as a direct challenge to the Islamic Republic and its supreme leader, who under the constitution is held to be God's representative on Earth.
In his sermon, Khamenei criticized some of Ahmadinejad's conduct before the June 12 vote, and he condemned the killing of students by pro-government loyalists this week. But he came down explicitly on the side of the hard-line incumbent. He described Ahmadinejad as hard-working, and said the president's views were closer to his than those of other politicians.
Khamenei and Ahmadinejad draw support from the same sources, including Basiji militiamen and the Revolutionary Guard.
"He cannot afford to sacrifice Ahmadinejad," said an analyst in Tehran, who spoke on condition of anonymity. "Over the last years Khamenei has teamed up with the Revolutionary Guard to build his power. For Khamenei and the Revolutionary Guard, Ahmadinejad is the best alternative."
Mousavi has become the head of a diverse movement that is pulling him forward as much as he's leading it. Reformists and moderates said they feared the movement would peter out without his leadership.
"The ball is now in Mousavi's court," said Nader Hashemi, a professor of Middle East studies at the University of Denver. "His leadership will now be tested. Will he stand firm and continue his nonviolent resistance, or will he compromise and sell out the democratic aspirations of millions of Iranians?"
Mousavi did not immediately respond to Khamenei's challenge.
The prime minister-turned-scholar remains under close supervision of authorities, has been denied access to state broadcasting facilities, and has seen his newspaper shut down.
"The positive aspect of this movement is that for the first time it's everyone," said an analyst in Tehran, who spoke on condition he not be named. "The election and the campaign [were] just an excuse to express all their problems. But on the other hand, the weakness of the movement is that it's disorganized. The main problem is that they crystallized all their anger around Mousavi. He became a hero. But in many ways the movement is ahead of Mousavi."
