The official Iranian news agency reported that intelligence and security agents in Tehran concluded that a Moussavi campaign office was used for “illegal gatherings, the promotion of unrest, and efforts to undermine the country’s security,” leading to speculation that Mr. Moussavi could be arrested. The news agency reported that “the plotters have been arrested.”

The government also stepped up its efforts to block independent news coverage of events all across the country. The government banned foreign news media members from leaving their offices, suspended all press credentials for the foreign press, arrested a freelance writer for The Washington Times, continued to hold a reporter for Newsweek and forced other foreign journalists to leave the country.

That made it difficult to ascertain exactly what happened when several hundred protesters tried to gather outside the Parliament building late in the afternoon. Witnesses said they were met by a huge force of riot police officers and Basij vigilantes, some on motorcycles and some in pickup trucks, armed with sticks and chains. Witnesses said people were trapped and beaten as they tried to flee down side streets.

“It was not possible to wait and see what happened,” said one witness who asked for anonymity out of fear of arrest. “At one point we saw several riot police in black clothes walk towards a group of people who looked like passers-by. Suddenly they pulled out their batons and began hitting them without warning.”

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Witnesses reported scenes of chaos and fear where riot police officers outnumbered demonstrators by about four to one. Many wore masks to conceal their identities. The Basijis stopped people to check their cellphones for video or pictures of the unrest.

“I saw one group of about 100 people who began chanting ‘Death to the dictator’ on one of the side streets,” said another witness who insisted on not being identified for fear of arrest. “The Basijis attacked them and beat them really bad.” Unconfirmed reports of bloodshed and at least one death flooded the Internet.

The authorities said they were moving to impose order and secure the rule of law. “I was insisting and will insist on implementation of the law,” Ayatollah Khamenei said on national television. “That means we will not go one step beyond the law. Neither the system nor the people will yield to pressure at any price.”

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The pressure exposed deep cracks in the opposition, but also sparked signs of entrenched resistance. Early in the day, Mohsen Rezai, a former head of the elite Revolutionary Guards and a presidential candidate, withdrew his charges of election fraud, saying that it was in the best interest of the country to drop the matter. His decision helped the government’s effort to cast the opposition as at war with the state.

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Then Mr. Moussavi, the defeated candidate who embodied the hopes of reformers, posted a notice on his Web site of a late afternoon rally in front of the Parliament, but he distanced himself from the action, saying it was not organized by the reform movement. It is not clear how far Mr. Moussavi, a former prime minister who is essentially an insider thrust into the role of opposition, would go to defy the system. He has not been seen since Thursday. So as the crackdown infuriates protesters, there is a greater gap with their ostensible leader, political analysts said.

“People in the street have been radicalized, and I do not believe that most of them would today subscribe to Moussavi’s avowed platform,” said a political analyst with years of experience in Iran.

Instead, Mr. Moussavi’s wife, Zahra Rahnavard, a former university dean, continued to rally the opposition movement. She posted a message on another Web site associated with her husband calling on the public to stand firm while criticizing the government for acting “as if martial law has been imposed on the streets.”

The head of Iran’s intelligence service announced that would-be terrorists, including two foreign reporters, were detained all over the country planning explosions and acts of sabotage, the official Iran news agency reported.

Iran’s defense minister, Brig. Gen. Mostafa Mohammad Najjar, pointed to recent military maneuvers in the Persian Gulf as proof that Iran could crush any foreign threat. The government’s harsh response, including the killing of at least 17 protesters, led to divisions among some conservatives who criticized the armed attacks on unarmed civilians. The mayor of Tehran, Mohammad Baqer Qalibaf, a former commander of the Iranian police, called on the government to authorize peaceful opposition rallies. And the speaker of Parliament, Ali Larijani, a longtime conservative, accused the Guardian Council, responsible for monitoring the elections, of bias and said most Iranians were suspicious of the election results.

Instead of heeding calls for moderation, the government has conducted one of the harshest crackdowns in its history. Dozens of former high-ranking officials have been jailed. The International Campaign for Human Rights in Iran reported Wednesday that about 240 people, including 102 political figures, were in jail. The government has said that it arrested 627 more people since the protests broke out.

Those arrested include officials who served from the founding of the Islamic republic in 1979, until Mr. Ahmadinejad’s election in 2005: Behzad Nabavi, a former deputy speaker of Parliament; Mohsen Aminzadeh, a key figure at the Intelligence Ministry for many years; Mostafa Tajzadeh, a deputy interior minister during the presidency of Mohammad Khatami; Mohammad Ali Abtahi, a vice president under Mr. Khatami; and Abdullah Ramezanzadeh, Mr. Khatami’s spokesman. They were all close to Mr. Khatami, then threw their support behind Mr. Moussavi.