Lately, the authorities seem to have singled out two groups in particular: journalists, including political and cultural reporters and editors, and women’s rights activists, who have years of experience in organizing and maintaining a movement in the face of a hostile government.

Image Iranian security officers on motorcycles surrounded antigovernment protesters on foot during clashes in Tehran on Dec. 27. Credit... Agence France-Presse — Getty Images

Iran’s leadership says it is determined to maintain control of the streets on the anniversary of the revolution, one of the most emotionally charged days in the Iranian calendar. In this smoldering political conflict, the opposition and the government have both tried to claim the mantle of the true heir to the revolution. Iranian officials often rely on large crowds of people  many of them paid by the government, the opposition charges  to prove the government’s legitimacy, and on Thursday the streets are likely to be filled with the state’s supporters as well as its opponents.

The pace of arrests, which soared in the summer, picked up again at year’s end after tens of thousands of demonstrators filled the streets around the country during the national observance of Ashura, a religious day of mourning observed by Shiite Muslims. At least 10 people were killed when government forces opened fire on unarmed protesters. But in scenes circulated around the world on the Internet, protesters were seen fighting back, chasing after government gunmen, blocking roads and burning government vehicles.

With those images clearly in mind, the government has moved aggressively to try to prevent a repeat. Experts say the officials have reason to be nervous. Even without calls from the principal opposition leaders, enormous crowds filled the streets on Ashura. This time those leaders, Mr. Moussavi and Mehdi Karroubi, have both issued calls to people to pour into the streets.

“There are signs of an exceedingly nervous security apparatus that is deeply concerned about the outcome,” said Hamid Dabashi, professor of Iranian studies and comparative literature at Columbia University.

The government response has been to try to intimidate, Iran experts and opposition leaders said. That has included imposing the death penalty on 11 prisoners, and hanging two. Five more death penalty cases are currently being prosecuted.

In the most recent crackdown, the government has rounded up scores of journalists, and not just those of the opposition. Reporters working for the semiofficial news agencies Mehr and ISNA have also been detained. The list of those arrested is a virtual Who’s Who of Iranian journalism, among them: Akbar Montajabi, political editor of the newspaper Etemad-e Melli; Ahmad Jalali-Farahani, the social affairs editor of Mehr; and Zeinab Kazemkhah, an arts and culture writer for ISNA. Other journalists, like Ahmad Zeydabadi and Masoud Bastani, arrested shortly after the election, have been sentenced to long prison terms in secret court proceedings.