"The regime has been visibly shaken by its own population and does not want to let this perception endure. That is why the media have become a priority target," RSF said.
"The Islamic republic of Iran now ranks alongside China as the world's biggest prison for journalists."
It said that jailed journalists were under pressure to make filmed confessions and that there were allegations of torture.
'Detained without charge'
Newsweek said that Maziar Bahari, who has been living in Iran for the past decade, had been "detained without charge by Iranian authorities and has not been heard from since".
"Newsweek strongly condemns this unwarranted detention, and calls upon the Iranian government to release him immediately," the New York-based weekly newsmagazine said in a statement.
"Mr Bahari's coverage of Iran, for Newsweek
and other outlets, has always been fair and nuanced, and has given full weight to all sides of the issues. He has worked well with different administrations in Tehran, including the current one."
Media coverage of the street protests in the capital, Tehran, has been heavily restricted after Iranian officials suggested that the foreign media were playing a role in the unrest.
The authorities have also blocked websites and mobile phones services since the protests started.
RSF said that a blogger known as the "Blogging Mullah" was among the detained, as well as a cartoonist, a TV producer, the publisher of several newspapers, a disabled former newspaper editor and a business reporter.
Iranian authorities told Jon Leyne, a correspondent for Britain's BBC news network, on Sunday that he had 24 hours to leave the country.
State radio reported that Leyne had been accused of "distortion of news regarding the Islamic Republic of Iran and particularly news pertaining to the election".
The semi-official Fars News Agency said he was accused of "ignoring neutrality in news" and "supporting rioters and trampling the Iranian nation's rights".'Interference'
The BBC has repeatedly accused Iranian officials of interferring with the satellite broadcasts of its Farsi-language news service in recent days.
"The satellite operator has traced the interference and has confirmed it is coming from within Iran. This interference is contrary to all international agreements for satellite usage to which Iran is a signatory," the BBC said in a statement.
The Dubai-based Al-Arabiya television channel has also been accused of "unfair reporting" and its Tehran bureau has been ordered to remain shut indefinitely.
"The authorities accuse Al-Arabiya of diffusing news that is not necessarily fair from their point of view," Mohammed al-Khateeb, the channel's editor-in-chief, said.
"They have ordered that we do not broadcast any news about Iran, saying Al-Arabiya in Dubai does not comply with what Al-Arabiya office in Tehran was ordered to do."
Authorities originally closed the office for one week from June 14 after criticising its handling of the elections.