The opposition may have little opportunity to keep momentum going within the limits of the law, and the international attention that appeared to bolster their morale could be waning. Also, Mousavi's Web site, his primary means for communicating with supporters, remained down on Saturday; an aide told the Associated Press Friday that the site had been hacked.

In one of the harshest statements from authorities since protests broke out after the June 12 election, Ayatollah Ahmed Khatami, a ranking cleric, said "Anyone who takes up arms to fight with the people, they are worthy of execution."

Those who disturbed the peace and destroyed public property were "at war with God" and should be "dealt with without mercy," he said Friday in a nationally televised sermon.

His call for merciless retribution for those who stirred up Iran's largest wave of dissent since the 1979 Islamic Revolution came as Mousavi slipped further from view.

Mousavi said he would seek official permission for any future rallies, effectively ending his role in street protests organized by supporters who insist he won the election.

Mousavi alleges he was robbed of victory through widespread and systematic fraud. The regime rejects the claim, refusing to consider new balloting, and on Friday, the Guardian Council — Iran's top electoral body — proclaimed the vote the "healthiest" held since the revolution.

Iran's supreme leader, Ayatollah Ali Khamenei, has ruled out a revote.

Since the election, opposition protesters repeatedly have clashed with security forces who arrested hundreds of people, including journalists, academics and university students. At least 17 people have been killed, in addition to eight members of the pro-government Basij militia, officials have said.

The demonstrations petered out this week under an ever-intensifying crackdown. Mousavi, meanwhile, has sent mixed signals to supporters, asking them not to break the law while pledging not to drop his challenge.

Amnesty International called the prospect of quick trials and capital punishment for some detainees "a very worrying development." It said Iran was the world's No. 2 executioner after China last year, with at least 346 known instances of people put to death. The group also called on the regime to release dozens of detained journalists it said faced possible torture.

As the protests dwindle amid intensifying official pressure, the opposition may suffer from a decline in international attention. The protests and violence dominated Western news broadcasts for nearly two weeks, with the reports substantially bolstered by videos gleaned from Internet sites and by commentary from social networking sites.

Such sites were a key pipeline for the opposition amid the tight restrictions on foreign media in the country.

But along with the diminished action on the streets in Iran, other stories have arisen to siphon away attention — especially the death of pop star Michael Jackson.

Television coverage of Iran's turmoil has fallen since Jackson's death Thursday; on the Twitter micro-blogging site, Iran remained among the most discussed topics, but fell below Jackson and comments about the movie "Transformers 2."

Heintz reported from Cairo; Associated Press writer Shaya Tayefe Mohajer in Cairo contributed to this report.