His organisation's legion of data warriors have embarrassed the US government by leaking diplomatic cables and shining a light on its soldiers' behaviour in Afghanistan and Iraq. Now Julian Assange is encouraging his WikiLeaks army to gang up on an unlikely new target: Unesco, the UN agency tasked with promoting human rights and protecting world heritage sites.

In what Unesco suggested was further evidence of founder Mr Assange's increasingly sensitive ego – which some critics have described as outright paranoia – WikiLeaks called on its online supporters to "occupy Unesco" out of anger at not being invited to a media conference at the agency's headquarters in Paris.

The whistleblowing website expressed "outrage" at not being granted a speaking slot on one of the panel discussions at the two-day event, entitled: "The Media World after WikiLeaks and News of the World." The event concludes today.

WikiLeaks claimed it had been banned from the event, organised by the World Press Freedom Committee. Yet key speakers yesterday included Mr Assange's lawyer, Geoffrey Robertson, who delivered a passionate defence of the website in a 15-minute speech, while Unesco pointed to emails in which it had encouraged the website to "attend the Conference and take part of the debate".

Mr Robertson said that he was not representing WikiLeaks, but that the website not having an official presence was "like Hamlet without a prince".

"They're making a big thing saying they're banned from the conference," Guy Berger, Unesco's director for freedom of expression and media development, said. "I think their frustration may be that they would like to be centre-stage. The door is open but it's not the entire stage and the entire focus of the conference that is open to them."

But WikiLeaks spokesman Kristinn Hrafnsson was adamant. "It is primarily a conference about the effects of WikiLeaks, and if you look at the key questions that were raised in the programme you cannot say that it is not about WikiLeaks, that would be absurd," he said.