The bloodshed, some of the heaviest in months, drew an especially harsh condemnation from one opposition leader, who compared the government to the brutal regime that was ousted by the Islamic Revolution three decades ago.
Monday's developments were sure to deepen antagonism between the government and the reform movement, which has repeatedly shown resilience in the face of repeated crackdowns since June's disputed presidential election.
Mahdi Karroubi, an opposition leader who ran in the June election, posted a statement on an opposition Web site asking how the government could spill the blood of its people on the Shiite sacred day of Ashoura. He said even the former government of the hated shah respected the holy day.
"What has really happened that (caused the ruling system) spilled the blood of people on the day of Ashoura and gets a group of savage individuals confronting people?" he told the Rah-e-Sabz Web site. The shah, who was overthrown in 1979, was widely hated, and comparing a rival to the shah is a serious, though common, insult in Iranian politics.
The government crackdown has attracted a growing chorus of international criticism. On Monday, Germany's foreign minister, Guido Westerwelle, condemned 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," he said. "The international community will watch and not look away."
Sunday's violence erupted when security forces fired on stone-throwing protesters in the center of Tehran. Opposition Web sites and witnesses said five people were killed, but Iran's state-run Press TV, quoting the Supreme National Security Council, said the death toll was eight. It gave no further details.
The dead included a nephew of chief opposition leader Mir Mousavi, according to Mousavi's Web site, Kaleme.ir. Police denied using firearms.
Opposition Web sites and activists said security forces raided a series of opposition offices on Monday, making at least seven arrests.
The Parlemannews site said three of Moussavi's top aides were rounded up, including his top adviser, Ali Riza Beheshti.
Security forces also stormed a foundation run by reformist former President Mohammad Khatami and arrested two people, a foundation official said, speaking on condition of anonymity because of fears of police reprisal. The Baran Foundation works to promote dialogue between civilizations.
In another move, 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.
The arrests could not be independently confirmed.
Some accounts of the violence Sunday in Tehran were vivid and detailed, but they could not be independently confirmed because of government restrictions on media coverage. Police said dozens of officers were injured and more than 300 protesters were arrested.
The street chaos coincided with commemorations of Ashoura, fueling protesters' defiance with its message of sacrifice and dignity in the face of coercion. The observance commemorates the 7th-century death in battle of one of Shiite Islam's most beloved saints.
Still, many demonstrators had not anticipated such harsh tactics by the authorities, despite police warnings of tougher action against any protests on the sacred day.
The clashes marked the bloodiest confrontation since the height of unrest in the weeks after June's election. The opposition says Ahmadinejad won the election through massive vote fraud and that Mousavi was the true winner.
The Dec. 20 death of the 87-year-old Grand Ayatollah Hossein Ali Montazeri, a sharp critic of Iran's leaders, has given a new push to opposition protests. Opposition leaders have used holidays and other symbolic days in recent months to stage anti-government rallies.
Iran is under pressure both from its domestic opposition within the country and from the United States and its European allies, which are pushing Iran to suspend key parts of its nuclear program.
U.S. National Security Council spokesman Mike Hammer, speaking in Hawaii Sunday, where U.S. President Barack Obama is vacationing, denounced Tehran's "unjust suppression of civilians."
Foreign Minister Carl Bildt of Sweden, which holds the rotating presidency of the European Union, expressed concern about the "increased repression" in Iran.