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“This is a battle between Khamenei and a generation that does not owe its political credentials to him,” Mr. Khalaji said.

Mr. Mousavi is a former prime minister and is backed by two ex-presidents — Mohammed Khatami and Ali Akbar Hashemi Rafsanjani.

Peter Ackerman, the founding chairman of the International Center on Nonviolent Conflict, said he did not know whether the stirrings in Iran represented a full-fledged colored revolution like those that have occurred in recent years in Ukraine, Lebanon and Georgia.

“Historically, these movements need to involve widespread support from all the key cultural and demographic elements in the society, and they need to have leaders who can sustain the pressure for months — not weeks, not days,” he said. “Once the heat is out for the moment, where will the movement be?”

Mr. Ackerman’s group in 2006 sponsored a workshop in the United Arab Emirates for Iranians on the strategic application of civil disobedience. Some of the participants were arrested later that year by Iran’s security services that accused them of trying to foment a velvet revolution in Iran.

Mr. Ackerman said the purpose of the workshop was “to review the strategic principles underlying successful civil-resistance movements.”

“Those principles are extremely relevant right now in Iran,” he said.

Suzanne Maloney, an Iran specialist at the Brookings Institution’s Saban Center, said the political chaos in Iran will make it far more difficult for the United States to engage with the country — at least in the short term.

Who ultimately emerges as president does matter, she added, noting that Iran was less repressive internally and less belligerent abroad when Mr. Khatami was president from 1997 to 2005.

Since Mr. Ahmadinejad was elected in 2005 with the apparent blessing of Ayatollah Khamenei, “we’ve seen creeping authoritarianism from the military and demagogic populism from Ahmadinejad,” she said.

The presidency “matters for the mind-set and opportunities of the Iranian people,” she said.

Even if Mr. Ahmadinejad prevails, Ms. Maloney and other specialists said, Iran has been irrevocably changed by the unprecedentedly open debate that preceded the election, the more than 80 percent voter turnout and the massive demonstrations that have followed.

“These are historic events unfolding,” she said. “We are very possibly at the brink of dramatic change in Iran. For the first time in 30 years, a very energized public is willing to risk its lives on the streets.”

• Eli Lake contributed to this report.