A U.S.-based rights group has called on Iran's government to end what it described as a nationwide crackdown on opposition activity after a disputed June 12 presidential election, Reuters reported.

Human Rights Watch said it had confirmed from sources across Iran the arrests of hundreds of opposition and reformist activists since street protests began on June 13 against official results of the vote.

The Interior Ministry said hardline President Mahmoud Ahmadinejad won the vote by a landslide, prompting days of demonstrations by supporters of defeated candidate Mirhossein Mousavi, who says the election was rigged.

"Those arrested include prominent political and religious leaders on the reforming wing of the political establishment as well as leading intellectuals, journalists, and students," Human Rights Watch said in a June 19 statement.

"The arrest of hundreds of protesters, including university students in Tehran and Mashhad, has been reported by unofficial Iranian internet news sites," it said.

Human Rights Watch said scores of reformist politicians, intellectuals and journalists were arrested on June 17 and 18 and also cited violent attacks by police and state-sponsored militias against largely peaceful demonstrators.

Other signs of the crackdown included attacks by security forces and state-sponsored militias on university student dormitories, the severe disruption of internet and mobile telephone communications, and restrictions on international and domestic media, it said.

Iranian police have blamed "bandits" for street unrest over the last week and the Intelligence Ministry said it has arrested a number of "main agents" behind the post-election violence.

Supreme Leader Ayatollah Ali Khamenei issued a strong warning on Friday to protest leaders that they would be responsible for any bloodshed.

"Peaceful protests are a fundamental right," said Sarah Leah Whitson, director of the Middle East and North Africa division of Human Rights Watch.

"The government needs to stop its harassment and intimidation of its critics, including peaceful demonstrators," she said.

Earlier this week, reformist sources told Reuters police had detained over 100 reformers, including a brother of former President Mohammad Khatami and leading reformist Mohammad Ali Abtahi. Police denied Khatami's brother had been arrested.

Separately, scores of protesters have been detained in various cities over the last week, according to Iranian media.

"There have ... been reports of violent attacks by security forces on demonstrators and students in the provincial towns of Shiraz, Isfahan, Tabriz, Bandar Abbas, and Mashhad," Human Rights Watch said.