Protesters hold up photos of opposition activists killed in clashes in Tehran. (Remy de la Mauviniere/Associated Press)

Opposition activists said Iranian security forces rounded up at least seven prominent activists on Monday, stepping up a crackdown on the country's pro-reform movement a day after the nephew of the chief opposition leader and seven other people were killed in anti-government protests.

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 ousted by the Islamic Revolution three decades ago.

Mahdi Karroubi, an opposition leader who ran in the June election, posted a statement on an opposition website 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 [the ruling system] spilled the blood of people on the day of Ashoura and gets a group of savage individuals confronting people?" he asked on the Rah-e-Sabz website. The shah, 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, German Foreign Minister Guido Westerwelle condemned the "brutal action" by security forces.

Sunday's violence erupted when security forces fired on stone-throwing protesters in the centre of Tehran. Opposition websites 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 website, Kaleme.org. Police denied using firearms.

At least 7 arrested

Opposition websites 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. 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 website. 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.

Obama condemns arrests

Iran's crackdown on activists drew a harsh response from U.S. President Barack Obama, who condemned "the violent and unjust suppression of innocent Iranian citizens."

"For months, the Iranian people have sought nothing more than to exercise their universal rights," he said. "Each time they have done so, they have been met with the iron fist of brutality even on solemn occasions and holy days."

Obama called on the Iranian government to respect the rights of its people and immediately release those who have been detained.