Organisation's founder says he is reliant on public opinion to rein in 'superpower that does not appear to be following rule of law'

WikiLeaks faces a "very aggressive" and secretive investigation by US authorities stung by a perceived loss of face following the release of thousands of secret American diplomatic cables, the organisation's founder, Julian Assange, said today.

Speaking to reporters outside Ellingham Hall, the Norfolk house at which he is staying on bail following his release from prison, Assange said WikiLeaks faced "what appears to be an illegal investigation ... certain people who are alleged to be affiliated to us have been detained, followed around, had their computers seized and so on".

He said he believed it was "80% likely" that the US authorities were seeking to prepare an attempt to have him extradited there to face charges of espionage.

He added that he was reliant on public opinion to rein in "a superpower that does not appear to be following the rule of law".

"I would say that there is a very aggressive investigation, that a lot of face has been lost by some people, and some people have careers to make by pursuing famous cases, but that is actually something that needs monitoring," he said.

He criticised the way Swedish authorities have sought to have him extradited to Sweden to face allegations of sexual assault – the reason he was held in jail for 10 days.

"That is something that actually needs monitoring, it needs scrutiny," he said. "We have seen this with the Swedish prosecutor in representations to the British government here, and the British courts say that it did not need to provide a shred of evidence – said this three times – and in fact has provided nothing, not a single shred of evidence in its extradition hearings, in the hearings that ended up putting me in solitary confinement for 10 days.

"Similarly, in the United States, what appears to be a secret grand jury investigation against me, or our organisation – not a single comment about what is actually going on."

The bulk of WikiLeaks' efforts were currently devoted to fending off various attacks, including technical assaults on its website, Assange said.

"Over 85% of our economic resources are spent dealing with attacks – dealing with technical attacks, dealing with political attacks, dealing with legal attacks, not doing journalism," he said. "And that, if you like, is attack upon investigative journalism."

Assange said he was worried about the prospect of being sent to the US, adding: "There have been many calls by senior political figures in the United States, including elected ones in the Senate, for my execution, the kidnapping of my staff, the execution of the young soldier Bradley Manning ... that's a very, very serious business.

"The United States has shown recently that its institutions seem to be failing to follow the rule of law. And dealing with a superpower that does not appear to be following the rule of law is a serious business."

US efforts to prosecute Assange appear to rely on connecting him to Manning, the presumed source of the leaked cables.

Assange, an Australian, was at pains today to stress his remove from Manning, referring to him as "a young man somehow embroiled in our publishing activities" and saying WikiLeaks did not know who its sources were.

Targeting him personally would not stop the work of WikiLeaks, Assange pledged. "People like to present WikiLeaks as me and my backpack. It is not true. We are a large organisation.

"It is resilient. It is designed to withstand decapitation attacks, and our publication rate actually increased during the time I was in solitary confinement."

Assange was held in jail because prosecutors argued that as a non-British national with no permanent ties to the country he was a potential absconder.

To satisfy the judge he had to post £200,000 in surety, provided by supporters, and agree to stay at Ellingham Hall, owned by his friend Vaughan Smith. He must wear a tag, observe a curfew and report to a police station daily.