Pro-reform Web sites and activists said the government also detained at least eight prominent opposition figures including a former foreign minister in an intensified crackdown that could fuel more violence of the kind that engulfed the center of Tehran on Sunday. The activity pushed the bitterly opposed camps beyond any immediate prospect of reconciliation or compromise.
Hardliners, including clerical groups and the elite Revolutionary Guard, issued statements urging the country's judiciary to take action against the opposition for violating Islamic principles and insulting the head of Iran's religious leadership, Supreme Leader Ayatollah Ali Khamenei.
In the bloodiest protests in months, groups of emboldened demonstrators on Sunday chanted slogans against Khamenei, casting aside a taboo on personal criticism of the leader. In outbursts of fury rarely seen in past street confrontations, they burned squad cars and motorcycles belonging to security forces who had opened fire on the crowds, according to witness accounts, opposition Web sites and amateur videos posted on the Web.
"I believe we are moving toward a more militarized and repressive confrontation. Things are going to get worse," said Ahmad Bakhshayesh, a political science professor at Tehran's Allameh Tabatabaei University.
IRNA, Iran's state-run news agency, said the bodies of five protesters, including the nephew of opposition leader Mir Hossein Mousavi, were being held pending autopsies. The family of the nephew, Ali Mousavi, alleged that he was shot by security forces or government-backed militiamen, and his funeral would likely galvanize another outpouring of opposition anger.
The nephew's brother, Reza Mousavi, earlier said the body was taken overnight from a Tehran hospital.
"Unfortunately, they have taken the body of my brother from the hospital, and however much we search, we can't find the body," Reza Mousavi had told the reformist Web site Parlemannews.ir.
Islamic tradition calls for bodies to be buried within 24 hours of death.
The opposition has alleged that Mousavi's nephew had received death threats in recent days and was shot by assassins who drove to his house. Reformists believe the killing was an attempt to pressure Mousavi to back down, and that the government took his nephew's body to prevent mourners gathering in the street for his funeral.
Iranian state television reported that eight people died in the violence in Tehran, a higher toll than the five deaths reported by some opposition Web sites. The television also cited the Health Ministry as saying 60 people were injured, and many had been released from hospitals after treatment.
Opposition Web sites reported 9 people had been killed, reported CBS News correspondent Elizabeth Palmer.
Independent confirmation of the casualties was virtually impossible because of state restrictions on media coverage of the upheaval that has gripped Iran since a disputed election in June.
Iranian authorities have said 300 people were arrested in the protests, but did not specify where they were detained. The opposition Jaras Web site said several hundred were arrested in Tehran, and a similar number were detained in the central city of Isfahan.
Tehran residents say limits on Internet access have been tightened since Sunday, and Iranians were unable to see opposition Web sites. Cell phone and text messaging services were sporadic. Communication problems are common around the time of demonstrations, likely a government bid to prevent negative publicity and disrupt coordination among protesters.
The Parlemannews.ir site said three Mousavi aides were detained Monday, including top adviser Ali Riza Beheshti.
Security forces also arrested two people in a raid on a foundation run by the reformist former President Mohammad Khatami, a foundation official said on condition of anonymity because of fears of police reprisal. The Baran Foundation works to promote dialogue between cultures.
Former Foreign Minister Ebrahim Yazdi and human rights activist Emad Baghi were arrested, according to the Rah-e-Sabz Web site. Yazdi, who served as foreign minister after the 1979 Islamic revolution, is now leader of the banned but tolerated Freedom Movement of Iran. One of his aides was also detained.
Bakhshayesh, the Tehran professor, said the best way to defuse the crisis was for Khamenei to ask Akbar Hashemi Rafsanjani, a former president close to the reformists, to mediate between the two sides. He said Khamenei's absolute support for President Mahmoud Ahmadinejad, who is accused of stealing the election from Mousavi through massive vote fraud, was proving costly for the supreme leader.
Mahdi Karroubi, an opposition figure who also ran in the election, asked how the government could spill the blood of its people during commemorations of Shiite Islam's most important observance, Ashoura. The observance commemorates the seventh-century death in battle of one of Shiite Islam's most beloved saints, and it conveys a message of sacrifice in the face of repression.
He told the opposition Rah-e-Sabz Web site that even the government of the shah, overthrown in the 1979 Islamic Revolution, respected the holy day. Comparing a rival to the shah is a serious, though common, insult in Iranian politics.
The government crackdown drew sharp criticism from the West, which is already locked in a dispute with Iran over its suspected efforts to develop nuclear weapons. Iran says its nuclear program is for peaceful purposes.
Speaking from Hawaii, where he is on vacation, President Barack Obama said the turmoil in Iran "is not about the United States or any other country. It's about the Iranian people and their aspirations for justice and a better life for themselves. And the decision of Iran's leaders to govern through fear and tyranny will not succeed in making those aspirations go away."
Germany's foreign minister, Guido Westerwelle, condemned what he called the "brutal action" by security forces.
"I am calling on those responsible in Tehran to do everything in order to avoid a further escalation of the situation and to end the violence," Westerwelle said.
Foreign Secretary David Miliband of Britain said it was "particularly disturbing" to hear reports of the crackdown by security forces on the sacred occasion of Ashoura. The French Foreign Ministry criticized what it described as arbitrary arrests and violence against demonstrators.
The Dec. 20 death of the 87-year-old Grand Ayatollah Hossein Ali Montazeri, a harsh critic of Iran's leaders, gave renewed momentum to opposition protests. Opposition leaders have used holidays and other symbolic days in recent months to stage anti-government rallies.
Also Monday, a Dubai television company said it had not heard from its correspondent in Iran since he went missing near his Tehran house on Sunday.
Dubai Media Inc. said it was in touch with Iranian officials about the fate of Reza al-Basha, a 27-year-old Syrian. Dubai Media is the government-owned parent of a handful of television stations in the emirate.