Iran's state broadcaster IRIB was hacked for 10 seconds on Thursday, state media reported, as the country prepares to mark the anniversary of its 1979 Islamic Revolution.
"During a period of 10 seconds, the faces and voices of hypocrites appeared on (our) Channel One," IRIB said, a phrase Iran's clerical rulers use to refer to exiled opposition group People’s Mujahideen Organisation of Iran (PMOI).
The PMOI - also known as the Mujahideen Khalq Organisation (MKO) - presents itself as an alternative to Iran's theocracy and is the main faction within the exiled opposition umbrella organization, the National Council of Resistance of Iran (NCRI).
During the hack, pictures of MKO leaders Maryam and Masoud Rajavi appeared on state TV and a man's voice could be heard chanting "Salute to Rajavi, death to (Iranian Supreme Leader Ayatollah Ali) Khamenei!," according to videos posted on social media.
Deputy IRIB chief Ali Dadi said the case was under investigation.
"Our colleagues are investigating the incident. This is an extremely complex attack and only the owners of this technology could exploit and damage the backdoors and features that are installed on the systems," Dadi told state TV channel IRINN.
"Similar disruptions happened to the Koran Channel, Radio Javan and Radio Payam," he added, referring to other state-affiliated broadcast channels.
In the past, the Islamic Republic has been targeted by a series of cyberattacks such as one last year in October that disrupted the sale of heavily subsidized gasoline.
Iran has said it is on high alert for online assaults, which it has blamed on arch-foes United States and Israel. The United States and other Western powers meanwhile have accused Iran of trying to disrupt and break into their online networks.
Iran will hold official celebrations in early February to commemorate the 43rd anniversary of the revolution that toppled US-backed Shah Mohammad Reza Pahlavi.