DUBAI (Reuters) - Iranian authorities have arrested administrators of more than 20 groups on the messaging app Telegram for spreading "immoral content", semi-official Fars news agency reported on Sunday, the latest detentions in a clampdown on freedom of expression.

In recent weeks, Iran's powerful hardline Islamic Revolutionary Guards Corps (IRGC) has rounded up a number of artists, journalists and U.S. citizens, citing fears of Western "infiltration".

Their campaign coincides with Iran beginning the implementation of a nuclear deal signed with world powers in July, which hardliners oppose for fear it will open up Iranian society to what they see as corrupting Western influences.

The Telegram groups were targeted for their "immoral content", police spokesman Saeed Montazer al-Mahdi was quoted as saying by Fars. He also said more than 100 "hackers" had been arrested in the past month, without giving more details.

Telegram's Chief Executive Pavel Durov said last month that Iranian authorities had demanded he hand over "spying and censorship tools", and temporarily blocked the app when he refused. He was later informed by Iran's Ministry of Information that the request was "not authorized by any higher authorities".

The IRGC announced the Telegram users' arrests last week, saying they had shared images and text "insulting to Iranian officials" as well as "satire and sexual advice". At the time, the judiciary denied any such arrests had occurred. It has not commented on Sunday's reports.

Last week, U.N. human rights investigators called on the authorities to cease arresting, harassing and prosecuting journalists and other activists to pave the way for free debate ahead of parliamentary elections in February.

Under the nuclear deal, Tehran agreed to curb its nuclear program in exchange for an easing of sanctions on its economy. Iran always denied Western suspicions it wanted to develop a nuclear weapon.

Social Media

Smartphone messaging applications are highly popular in Iran, where half of the population is aged 24 or younger.

The Islamic Republic has some of the strictest controls on Internet access in the world, but its blocks on social media platforms such as Facebook, Twitter and YouTube are routinely bypassed by the tech-savvy.

Several branches of the security establishment monitor content, including the IRGC. Infractions on social media can result in harsh sentences from the hardline judiciary.

Last year, 11 people were arrested by the IRGC for insulting Ayatollah Ruhollah Khomeini, the Islamic Republic's founder, on messaging services Whatsapp, Tango, Viber and Telegram.

Telegram, which was launched in 2013, has become popular among activists and ordinary Iranians because it is seen as being more secure than its rivals.