Iran also accused the U.S. and Britain of fomenting the recent violence, threatening to "slap" Britain in the face as it summoned the British ambassador to an urgent meeting. Clashes on Sunday left at least eight people dead in a confrontation that has become an increasingly bitter and violent.
President Mahmoud Ahmadinejad shrugged off Sunday's protests as "a play ordered by Zionists and Americans" and criticized Barack Obama and Britain for allegedly supporting the protesters.
"The Iranian nation has witnessed this sort of play many times," Ahmadinejad said, according to the state IRNA news agency.
Government supporters held rallies in at least three cities on Tuesday, many protesting against the opposition and its leaders.
State Television broadcast pictures of the huge pro-government rally - the crowd calling for "opposition protesters to be executed," reports CBS News correspondent Elizabeth Palmer.
Opposition Web sites reported about 10 new arrests, and those taken into custody included the sister of Shirin Ebadi, who won the 2003 Nobel Peace Prize for her human rights efforts in Iran.
The new arrests, along with the tough criticism of the U.S. and Britain, added to rising tensions with the West, which is threatening to impose tough new sanctions over Iran's suspect nuclear program and has criticized the violent crackdown on anti-government protesters.
"The brutality of Iran's crackdown on dissent, coupled with the rejection of the U.N. nuclear agreement will make a new round of Security Council sanctions likely after the New Year," CBS News Foreign Affairs Analyst Pamela Falk reports, adding that "Israel's reported assessment today that Iran will have a nuclear bomb by 2011 makes the instability in Iran even more volatile."
Noushin Ebadi, a medical professor in Tehran, was arrested at her home by four intelligence agents late Monday and sent to prison, according to a statement issued by the Nobel laureate. It said authorities had "repeatedly summoned" Noushin to get her to persuade Shirin to drop her civil rights campaign.
The Nobel laureate has stayed outside of Iran since a day before the June elections. She told The Associated Press in a phone interview from London that Iranian authorities were trying to punish her by arresting her sister.
She had called her sister Monday and said that she was being punished because of the conversation.
"She was warned not to contact me," Ebadi said. "She is detained for the sake of me. She was neither politically active nor had a role in any rally."
The opposition Greenroad Web site also reported additional arrests, among them opposition leader Mir Hossein Mousavi's brother-in-law, Shapour Kazemi, and Mashallah Shamsolvaezin, a journalist who frequently criticizes the government. Others included the son of a prominent ayatollah, a reporter for the semiofficial ILNA news agency, and several activists. Mousavi's nephew was among those killed this week.
Iranian security forces also limited the movements of leading opposition figure Mahdi Karroubi by refusing to protect him when he leaves his home.
His son, Taghi Karroubi, told the AP by telephone that guards assigned to his father by Iranian police on Monday stopped providing security for him when he goes out, apparently under police orders. Police have for years provided leading opposition figures with security.
Without the guards, he said, his father cannot go outside safely and is under a "quasi-house arrest." If Karroubi leaves unprotected, he risks attack by hardline government supporters. His car was attacked on Saturday when he went out, and assailants shattered his front windshield.
Karroubi and Mousavi were the two defeated reformist candidates in the disputed June 12 presidential election, which set off the worst unrest in Iran since the 1979 Islamic Revolution.
There was no serious violence reported Tuesday, but the Greenroad Web site said students and security forces clashed at the Azad University's science department in Tehran.
It cited witnesses as saying the students were later "locked down" inside the building while pro-government Basij militiamen threatened to arrest those who dared to leave the premises.
Sunday's clashes were the worst since the aftermath of June's disputed presidential election. In outbursts of fury rarely seen in past street confrontations, protesters burned squad cars and motorcycles belonging to security forces who had opened fire on the crowds, according to witness accounts, opposition Web sites and amateur videos posted on the Web.
The exact death toll from Sunday's violence remains unclear. The government had said eight people were killed, but on Tuesday, Tehran's chief prosecutor said he was investigating only seven deaths.
One reason for the confusion is that the government hasof five slain protesters, including Mousavi's nephew, in what appears to be an attempt to prevent activists from using their funerals as a platform for more demonstrations. The bodies remained at a coroner's office Tuesday while the government said it was still conducting autopsies.
Independent confirmation of the casualties has been virtually impossible because of state restrictions on media coverage.
At a news conference, Foreign Ministry spokesman Ramin Mehmanparast said the deadly clashes in Tehran were the work of a tiny minority, and he accused outside countries, including the U.S. and Britain, of "miscalculating" by siding with the protesters.
"Some Western countries are supporting these sort of activities. This is intervention in our internal affairs. We strongly condemn it," he said.
He gave no further details, but Foreign Minister Manouchehr Mottaki threatened unspecified retaliation against Britain.
"If this country does not stop its prattling, it will receive a slap in its face," he said during a news conference with his Somali counterpart. The quote was posted on the Web site of state TV.
Britain, France, Germany and the U.S. have criticized Iran's violent response to the protests. On Monday, President Obama praised "the courage and the conviction of the Iranian people" while condemning Iran's Islamic government for attacking demonstrators with "the iron fist of brutality."
Traveling with Obama in Hawaii, U.S. National Security Council chief of staff Denis McDonough also said the White House was reaching out to international partners to build support for a new round of sanctions against Iran.
The sanctions are to punish Iran for its refusal to halt uranium enrichment and accept a U.S.-backed plan to curb its nuclear program. The West suspects Iran is trying to build a nuclear bomb - a charge Tehran denies.