Iranian state television reported that more than 15 people were killed in protests, 10 of whom were members of "anti-revolutionary terrorist" groups.The other five who died during Sunday's fierce clashes in the Iranian capital were killed by "terrorist groups," the report said, without elaborating.
"In a suspicious act, five people were killed by terrorist groups and the ministry of intelligence announced that more than 10 of the known members of anti-revolutionary terrorist groups were killed," the state television website said.
An opposition leader earlier criticized Iran's hardline rulers for killing innocent people during a religious festival, a reformist website reported.
According to opposition websites the unrest had spread to other parts of Iran, including the holy city of Qom, Shiraz, Isfahan, Najafabad, Mashhad and Babol.
The reports could not be independently verified because foreign media are banned from covering protests.
The White House condemned the "unjust suppression" of civilians by the Iranian government and said the United States was on the side of protesters.
Canada condemned what it called "brutal violence" used by Iranian authorities against protesters and urged restoration of human rights in the country.
While German Chancellor Angela Merkel slammed the "unacceptable" crackdown on pro-opposition demonstrators and pressed Tehran to respect civil rights.
The killings showed that the confrontation between the opposition and the clerical establishment had entered a volatile phase, in which the security forces appeared determined to stamp out the pro-reform movement.
The disputed re-election of hardline President Mahmoud Ahmadinejad has plunged Iran into its biggest internal crisis since the 1979 Islamic revolution, exposing deepening divisions in its ruling elite and setting off a wave of protests that the opposition says left over 70 people dead.
Officials say the death toll was half that number.