The Iranian state broadcaster, Irib, has been accused of faking dramatic footage of an anti-government protester setting fire to a portrait of Ayatollah Khomeini, Iran's revered revolutionary leader.

The images caused shock and incredulity when broadcast and have now provoked a row among the country's Islamic leadership, with opposition leaders, senior clerics and the institution charged with preserving Khomeini's works accusing Irib of desecrating the late ayatollah's memory. Hundreds of theological students staged a rally in Tehran yesterday in protest at the "insult".

The alleged incident was screened repeatedly last Monday after a day of clashes between security forces and opposition demonstrators in Tehran and other cities. Protesters used the occasion of students' day to renew opposition to President Mahmoud Ahmadinejad, whose disputed re-election in June continues to send shock waves through Iran's political system.

Film of them setting alight Khomeini's portrait was calculated to convey the message that they were bent on reversing the 1979 Islamic revolution. The footage also showed pictures of Ahmadinejad and Ayatollah Ali Khamenei, Iran's supreme leader, being burned.

But sceptics pointed out that the face of the supposed protester was not shown and there appeared to be no witnesses in the shot. Portraits of Ahmadinejad and Khamenei have been defaced, but there had been no known previous instances of attacks on pictures of Khomeini.

Mir Hossein Mousavi and Mehdi Karroubi, the two defeated reformist candidates in the presidential election, claimed the images had been doctored.

"I am sure the students have never gone over such boundaries, because we all know they love the imam [Khomeini] and are prepared to sacrifice their lives for his goals," Mousavi told the newspaper Jomhouri Eslami.

Karroubi said in a statement on his website, Tagheer, that Irib was meant to be above politics, but "some in this organisation believe they can justify cracking down by using the name and reputation of the imam".

Even more damagingly, the Institution for Publishing Khomeini's Works said it knew of no instances of demonstrators desecrating his image. "Even if we assume that an ill-intentioned and anonymous person did this, expanding (the footage) to make it look as if the overwhelming atmosphere was in line with this incident is a clear lie," the institution's vice-president, Hamid Ansari, wrote in an open letter.

Irib insists its report was genuine. The broadcaster's vice-president, Ali Darabi, claimed that foreign broadcasters had aired the incident in even greater detail.