Iran arrested at least 10 leading opposition figures yesterday, a day after eight people were killed in anti-government protests that erupted during a Shiite Muslim religious festival, an opposition website said.

The Norooz opposition website said three advisers to opposition leader Mirhossein Mousavi had been detained, along with seven pro-reform politicians.

Jaras opposition website said police fired teargas yesterday to disperse Mousavi supporters who gathered to express their condolences over his death of his nephew, among those killed during Sunday's protests.

Iran's Supreme National Security Council said eight people were killed across Iran. The Health Ministry said more than 60 people had been injured in Tehran.

The deaths and scale of confrontations may signal a volatile new phase in which security forces loyal to Supreme Leader Ayatollah Ali Khamenei might try to crush the reformist movement.

U.S. President Barack Obama, on vacation in Hawaii, strongly condemned what he said was the "iron fist of brutality" to quell the protests and demanded the immediate release of those who had been detained.

"The United States joins with the international community in strongly condemning the violent and unjust suppression of innocent Iranian citizens," Obama said. "The United States stands with those who seek their universal rights."

German Chancellor Angela Merkel also called on Iran to respect "the right to free speech through peaceful demonstrations."

There were no reports of protests yesterday after opposition websites said tens of thousands of people took to the streets in cities across Iran during Sunday's Ashura festival.

The Web reports cannot be independently verified because foreign media are banned from directly covering protests. The cellphone text messaging system also was down in Tehran yesterday.

State television said unknown assailants killed Mousavi's nephew, Ali Habibi Mousavi Khamene. A Mousavi ally described his death as martyrdom.

"A group of Mousavi supporters have gathered in front of Ebn-e Sina hospital where his nephew's body was kept. ... Police fired teargas to disperse them," the Jaras website reported.

A moderate website said the body of Mousavi's nephew was missing from the hospital.

The official IRNA news agency denied the report, saying his body had been kept for "further investigation."

Clashes were expected at the funeral ceremony.

Iran's Revolutionary Guards, who helped to quell protests after June's disputed presidential vote, called on the judiciary to "firmly" encounter any protesters.

"The horrible insult to Ashura ... is unacceptable. ...We call for firm punishment of those behind this obvious insult," the Guards said in a statement.

Sunday's violence flared up across Iran. A Western diplomat in Tehran said Iran's leadership was under great pressure, but showed no sign of losing its grip.

Jaras said the unrest erupted in Qom, Shiraz, Isfahan, Najafabad, Mashhad and Babol, as well as Tehran.