TEHRAN, Iran - Iranian security forces intensified their crackdown on anti-government supporters yesterday, arresting relatives of the country's Nobel laureate and the main opposition leader, and limiting the movement of another top opposition leader.

Iran also accused the United States 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 increasingly bitter and violent.

President Mahmoud Ahmadinejad shrugged off Sunday's protests as "a play ordered by Zionists and Americans," and criticized President 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 yesterday, many protesting against the opposition and its leaders.

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.

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 that 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 said 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, said 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 yesterday, but the Greenroad Web site said that students and security forces clashed at the Azad University's science department in Tehran.