Iran's Islamic Revolution Guard Corps (IRGC) says it has arrested an exiled journalist for allegedly using a messaging app to stir up dissent.
Ruhollah Zam was "guided into the country" as part of a "complicated intelligence operation", the IRGC said.
It is not clear where or how Mr Zam, who was said to be living in Paris, France, was detained.
Mr Zam, 46, the son of reformist cleric Mohammad Ali Zam, has denied any links with foreign intelligence agencies.
He ran the Amadnews website, a popular anti-government online forum which Iran accuses of inciting the nationwide protests of 2017-18.
The news network, which has around 1.4 million followers on encrypted messaging app Telegram, shared videos of protests and damaging information about Iranian officials.
It was shut down by the Iranian government, which said it was fomenting an "armed uprising", but later reopened under a different name.
Mr Zam's arrest, announced on state-owned TV in Iran, was hailed as a victory over Western intelligence services.
The IRGC alleged that Mr Zam was "under the guidance" and protection of intelligence services in France, Israel and the US.
But using "modern intelligence methods and innovative tactics", the IRGC said it was able to "deceive" foreign services and arrest Mr Zam.
Long in the crosshairs
Daniel Amir, BBC Monitoring
Ruhollah Zam is best known as one of the main figures behind the controversial Telegram channel and website Amadnews.
The channel, whose name is the Persian acronym for "Knowledge, Struggle and Democracy", says its mission is "spreading awareness and seeking justice" in Iran. The channel's output is critical of the Islamic Republic, and has been known for publishing confidential information on figures in the ruling establishment.
Mr Zam has long been in the crosshairs of Iran's elite Islamic Revolution Guards Corps (IRGC). Mr Zam was imprisoned following the disputed 2009 presidential election and left Iran shortly after, eventually settling in France.
Iranian authorities have repeatedly accused him of being supported by foreign intelligence agencies.
In March of this year, Iranian state television aired a documentary showing how Mr Zam's activity was being closely monitored by Iran's intelligence organisations.
Mr Zam's Amadnews channel came to prominence during protests that started in December 2017.
In a country where other social media sites are banned, Amadnews played a key role in spreading information about the protests.
In its statement, the IRGC described Mr Zam as "one of the main people of the enemy's media network and psychological warfare".
Iran's IRGC was set up 40 years ago to defend the country's Islamic system, and to provide a counterweight to the regular armed forces.
It has since become a major military, political and economic force in Iran, with close ties to the Supreme Leader, Ayatollah Ali Khamenei, and many other senior figures.