An Iranian opposition website said three advisers of opposition leader Mirhossein Mousavi were detained today, a day after at least five people were killed in Tehran in clashes between security forces and opposition backers.
The Jaras website had earlier reported the arrests of four opposition politicians.
An opposition leader today accused Iran's hardline rulers of killing innocent people during a religious festival that ignited into a day of anti-government protests.
Iran's state English-language Press TV, citing the Supreme National Security Council, said eight people were killed in yesterday's clashes between security forces and demonstrators.
Police had earlier reported five dead in violence in Tehran, the first such fatalities since the street protests that flared immediately after a disputed June 12th presidential election.
"What has happened to this religious system that it orders the killing of innocent people during the holy day of Ashura?" asked moderate cleric Mehdi Karoubi, who came fourth in the election, in a statement posted on the Jaras website.
Opposition websites said police fired on protesters in Tehran. Eight people were killed in the capital and other cities when tens of thousands of people took to the streets, they said.
The deaths and scale of confrontations may signal a volatile new phase in which security forces loyal to Supreme Leader Ayatollah Ali Khamenei seek to crush the reformist movement.
"These are the hardest clashes we've seen since June," said a Western diplomat in Tehran, adding that bitterness over the deaths could spark new protests and a harsh state reaction.
He said Iran's leadership was under great pressure but showed no sign of losing its grip over the security apparatus.
Among the dead yesterday was a nephew of Mr Mousavi. State television said unknown assailants had killed Ali Habibi Mousavi Khamene, whose death was described as martyrdom by an ally of Mousavi.
Police said the "suspicious deaths" were under investigation and that 300 protesters had been arrested. They said dozens of security men had been injured in the running street clashes.
State television said members of an exiled opposition group, the Mujahideen Khalq Organisation, were among those arrested.
It quoted a senior police official as saying the security forces had not used weapons, apparently meaning firearms.
Reuters