Amateur video accounts showed at least one large protest gathering, on Shirazi Street, though it was unclear how long it lasted.

But in the network of Internet postings and Twitter messages that has become the opposition’s major tool for organizing and sharing information, a powerful and vivid new image emerged: a video posted on several Web sites that showed a young woman, called Neda, her face covered in blood. Text posted with the video said she had been shot. It was not possible to verify the authenticity of the video.

The Web site of another reformist candidate, Mehdi Karoubi, referred to her as a martyr who did not “have a weapon in her soft hands or a grenade in her pocket but became a victim by thugs who are supported by a horrifying security apparatus.”

Accounts of the election’s aftermath in the state-run press suggested that the government might be laying the groundwork for discrediting and arresting Mr. Moussavi. IRNA, the official news agency, quoted Alireza Zahedi, a member of the Basij militia, as saying Mr. Moussavi had provoked the violence, sought help from outside the country to do so and should be put on trial. The Fars news agency quoted a Tehran University law professor as saying that Mr. Moussavi had acted against “the security of the nation.” State television suggested that at least some of the unrest was instigated by an outlawed terrorist group, the Mujahedeen Khalq, which does not have a strong following in Iran.

Mr. Moussavi was not seen in public on Sunday but showed no sign of yielding. In his Web posting, he urged followers to “avoid violence in your protest and behave as though you are the parents that have to tolerate your children’s misbehavior at the security forces.”

He also warned the government to “avoid mass arrests, which will only create distance between society and the security forces.”

Image Iranian protesters covered their faces to protect themselves from tear gas during clashes with police in Tehran on Saturday. Credit... Ali Safari/Agence France-Presse — Getty Images

The moves against members of Mr. Rafsanjani’s family were seen as an attempt to pressure him to drop his challenge to Ayatollah Khamenei  pressure that Mr. Rafsanjani’s son, Mehdi Rafsanjani, said he would reject.