"During a period of 10 seconds, the faces and voices of hypocrites appeared on (our) Channel One," IRIB said, a phrase it often uses to refer to exiled opposition group People's Mujahideen Organisation of Iran (PMOI).
The PMOI - also known as the Mujahideen Khalq Organisation (MKO or MEK) - presents itself as an alternative to Iran's theocracy and is the main faction within the exiled opposition umbrella organisation, the National Council of Resistance of Iran (NCRI).
"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," deputy IRIB chief Ali Dadi told the 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.
A clip of the incident seen by the Associated Press showed the faces of MKO leaders Massoud Rajavi and his wife Maryam Rajavi suddenly superimposed on the channel's regular 3pm news programming.
A man's voice chants "Salute to Rajavi, death to (supreme leader) Khamenei".
Then, a speech from Rajavi briefly plays over the images.
He can be heard saying, "Today, we still honour the time that we declared death to the reactionary. We stood by it...".
Massoud Rajavi has not been seen publicly in nearly two decades and is presumed to have died.
Maryam Rajavi now runs the MKO.
In the past, the Islamic Republic has been targeted by a series of cyber attacks such as one last year in October that disrupted the sale of heavily subsidised petrol.
Iran has said it is on high alert for online assaults, which it has blamed on arch-foes United States and Israel.
The US and its allies meanwhile have accused Iran of trying to disrupt and break into their online networks.
with reporting from AP
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