Iran arrested at least 10 leading opposition figures on Monday, 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 on Monday 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," said Mr. Obama. "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 on Monday 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 was also down in Tehran on Monday.
State television said unknown assailants killed Mr. 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 on Monday the body of Mr. 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."
"We cannot hold a funeral until my brother's body is found," said another of Mr. Mousavi's nephews, according to Parlemannews website. 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, carried by the semi-official Fars news agency.
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 the cities of Qom, Shiraz, Isfahan, Najafabad, Mashhad and Babol, as well as Tehran.
"These are the hardest clashes we've seen since June," he said, referring to demonstrations after the June 12 vote, adding that bitterness over the deaths may spark fresh protests and a harsh state reaction.
Political turmoil has convulsed Iran since the re-election of President Mahmoud Ahmadinejad in the vote his opponents said was fraudulent, a charge the authorities deny.
Heavy security measures eventually quelled the first explosion of mass protests that plunged Iran into its biggest internal crisis since the 1979 Islamic revolution.
But the opposition has been using state-organized rallies to revive anti-government protests.
The opposition says more than 70 people were killed in the early protests. Officials say the toll - including members of a pro-government Islamic militia - was less than half that.
Opposition websites said police fired on protesters in Tehran on Sunday, saying eight people were killed in the capital and other cities. Police denied the claim.
Police said the "suspicious deaths" were under investigation and that 300 protesters had been arrested, adding dozens of security men had been injured in street clashes.
The Intelligence Ministry said members of an exiled opposition group, the Mujahideen Khalq Organization, were among those arrested.
Jaras said opposition politician Ebrahim Yazdi, leader of the banned Freedom Movement and foreign minister in Iran's first government after the 1979 Islamic revolution that overthrew the U.S.-backed shah, was detained early on Monday at his home.
Another opposition leader Mehdi Karoubi accused Iran's hard-line rulers of killing innocent people.
"What has happened to this religious system that it orders the killing of innocent people during the holy day of Ashura?" asked moderate cleric Karoubi, who came fourth in the election, in a statement posted on Jaras.
A hard-line cleric portrayed the unrest as a foreign-backed attempt to undermine the Islamic state.
"This is a plot against the Islamic Republic of Iran ... The judiciary should punish those behind Sunday's events," said Ahmad Khatami, Fars reported.
Hardliners in different cities staged pro-government rallies, condemning the "sedition by rioters" during Ashura, state television reported.