Everything about this is odd. Julian Assange, the founder, director, frontman, guiding spirit of global whistleblowing service WikiLeaks looks a bit odd for a start. Tall, cadaverous, dressed in ripped jeans, brown jacket, black tie, battered trainers. Somebody says he looks like Andy Warhol with his prematurely white hair, but I can't remember who, which will bother the hell out of him because accuracy is everything. He detests subjectivity in journalism; I fear that part of him detests journalists, too, and that WikiLeaks – which describes itself as an "uncensorable system for untraceable mass document leaking" – is essentially a way of cutting out subjectivist idiots such as me.

If Assange was producing this article, he would post the rambling hour-and-a-half-long talk he delivers at the Centre for Investigative Journalism's summer school at London's City University online, plus the 10 minutes we spend talking on the way to a restaurant – I almost get him run down by a speeding BMW, which would probably have changed the course of investigative journalism – and the additional 20 minutes of chat in the restaurant before it's politely suggested I've exhausted my time. "When you're dealing with any secondary sources [about me], be extremely careful," he says as we walk, even picking holes in a recent New Yorker piece, enormously long, detailed, no doubt majestically fact-checked, but in which the writer makes an assumption about one of his supporters based on little more than the T-shirt she is wearing.

"Journalism should be more like science," he tells me in the restaurant. "As far as possible, facts should be verifiable. If journalists want long-term credibility for their profession, they have to go in that direction. Have more respect for readers." He likes the idea of a 2,000-word article backed by 25,000 words of source material, and says there is no reason why you can't provide that on the internet. Come to think of it, I'm not sure that car was a BMW, or even that it was speeding.

Assange unveiled wikileaks.org in January 2007 and has pulled off some astonishing coups for an organisation with a handful of staff and virtually no funding. It has exposed evidence of corruption in the family of former Kenyan president Daniel arap Moi, published the standard operating procedures for the Guantánamo Bay detention centre, even made public the contents of Sarah Palin's Yahoo account. But what has really propelled WikiLeaks into the media mainstream is the video it released in April of a US helicopter attack in Baghdad in July 2007, which killed a number of Iraqi civilians and two Reuters personnel, Saeed Chmagh and Namir Noor-Eldeen.

The video, posted in a 39-minute unedited version and as an 18-minute film called Collateral Murder, gives a chilling insight into US military attitudes: sloppiness in identifying targets (the helicopter pilots mistook the Reuters employees' cameras for weapons), eagerness to finish off a grievously wounded man as he attempts to crawl to safety, and lack of concern even for two children in a van that arrives to pick up the bodies and is immediately attacked. "It's their fault for bringing their kids to a battle," says one of the pilots. "That's right," replies his colleague matter-of-factly. This, though, is one of the most one-sided battles you will ever witness. Very few cameras can bring down a helicopter gunship.

My thesis, soon to be exploded by Assange along with pretty well everything else I have predetermined on the basis of what I have read about him, is that this remarkable video is a transformative moment for WikiLeaks. But just before I can put that to him, a handsome, bearded student who was at the talk springs forward. "Julian, before you go, can I just shake your hand," he says, "because I really love what you do and you're like a hero, you really are." They shake hands. The icon and the acolyte. The Warhol parallel becomes ever stronger: Assange as impresario of a new form of news.

So the thesis. "Did the April video change everything?" I say. This is a rhetorical question, because I am quite sure it must have. "No," he says. "Journalists always like an excuse for why are they talking about something now when they didn't talk about something a week ago. They always like to say something is new." He does, though, accept that the scope of WikiLeaks is expanding rapidly. At the beginning of his talk, he said his head was "full of so many things at the moment", as if to excuse the faltering, unstructured nature of his presentation. What things? "We have been trying to raise funds for the past six months," he says, "so we've been doing very few releases and now we have an enormous queue of submissions that has piled up. We're working on those and working on engineering systems to speed up our publishing pipeline."

WikiLeaks has just five full-time staff and about 40 others who, he says, "very frequently do things", backed by 800 occasional helpers and 10,000 supporters and donors – an amorphous, decentralised structure, which might become the model for many media organisations in the future, as what might be called "journalism factories" become both outmoded and unfinanceable. This is a delicate moment in the development of what Assange prefers to think of as a "movement". "We have all the problems that a growing startup organisation has," he says, "combined with an extreme adversarial environment and state spying."

The danger of penetration by the security services is acute. "It makes it hard to get new talent quickly," he says, "because everyone has to be checked out, and it makes internal communication very difficult because everything has to be encrypted and security procedures put in place. And we also have to be ready to respond to lawsuits." On the plus side, the recent fundraising drive produced $1m, mostly from small donors. Large trusts, though, have steered clear of WikiLeaks because of political suspicions, worries about the legality of posting leaked material on the internet, and the common failing that western-based funding bodies are happy to underwrite exposés of malpractice in the developing world but less willing to look into the murky corners of so-called first world countries.

Is WikiLeaks the journalistic model for the future? He gives a characteristically lateral answer. "All over the world the barriers between what is inside an organisation and outside an organisation are being smoothed out. In the military, the use of contractors means that what is the military and what is not the military is smoothed out. Newswise, you see the same trend – what is the newspaper and what is not the newspaper? Comments on websites from the general public and supporters . . . " His point trails away, so I press him to make a prediction about the shape of the media in a decade or so from now. "For the financial and specialist press, it'll still look mostly the same – your daily briefing about what you need to know to run your business. But for political and social analysis, that's going to be movements and networks. You can already see this happening."

Assange has to be careful about his personal security. Bradley Manning, a 22-year-old US army intelligence analyst, has been arrested and charged with allegedly giving WikiLeaks the footage of the Baghdad attack, and the US authorities believe the organisation has another video of an attack on the Afghan village of Granai in which many civilians were killed. There have also been disputed reports that WikiLeaks may be holding 260,000 classified diplomatic cables, and the US authorities have been quoted as saying they want to interview Assange about all this material, publication of which would they say breach national security. Some sources with links in the intelligence agencies have warned him he is in danger and advised him not to travel to the US. He refuses to confirm that Manning was the source of the Baghdad video, but says whoever did leak it was "a hero".

At the talk I heard a man close to me say to his neighbour: "Do you think there'll be spooks here? The US are after him, you know." And of course it's possible. But giving a public talk to 200 students in the centre of London does not suggest someone who is in fear of extraordinary rendition. On the other hand, the organiser of the lecture tells me Assange tends not to stay in the same place two nights in a row. So is he taking the threats seriously? "When you first get them, you must take them quite seriously. Some very senior people advised me that there were significant problems, but there's a clarity now. The public statements from the [US] state department have mostly been reasonable. Some statements made in private have not been reasonable, but the demeanour of those private statements has changed over the past month and have become more positive."

Assange, despite his faltering manner, exudes self-confidence, immodesty even. When I ask him whether the rapid growth and increasing significance of WikiLeaks surprises him, he says no. "I was always confident the idea would succeed, otherwise I wouldn't have spent my time on it or asked other people to spend their time on it." He has spent a good deal of that time recently in Iceland, where freedom of information is protected and he has high-level supporters. It was here that the complex work of decrypting the video of the Baghdad attack was done. But he says he has no real base. "It's just like a war correspondent, I'm everywhere," he says. "Or like anyone setting up a multinational corporation, where you go visit all the regional offices. We have supporters in many countries."

Assange was born in Queensland in 1971 into what sounds a highly unconventional family – here one is relying on those secondary sources he warned me about, and it really would be useful to see the documentation. His parents ran a touring theatre company, and he went to 37 different schools (though some accounts suggest his mother thought school encouraged deference to authority, so educated him mainly at home). His parents divorced, his mother remarried, there was a bustup with her new husband, which led to her, Julian and his half-brother going on the run. It all sounds too Warholian to be true, but I suppose we have to trust it. There is no time to ask him for his life history, and I don't suppose he'd be very interested to tell it if there was. His replies generally are brief and a little hesitant, and when I ask him whether there is anything that WikiLeaks wouldn't publish he says, "That isn't an interesting question," in his soft Australian accent, and leaves it at that. Assange is not someone who feels the need to fill dead air.

He fell in love with computers in his teens, became a skilled hacker and formed a group called International Subversives, which broke into US defence department computers. He married at 18, and he and his wife soon had a son, but the marriage broke down and he fought a long custody battle, which, it is said, entrenched his dislike of authority. There are also suggestions he felt some people in the government had been conspiring against him. So we have a neat journalistic picture: computer expert with two decades of hacking experience, hostility to authority, conspiracy theorist. Setting up WikiLeaks in his mid-30s looks like an inevitable move.

"That's more a journalist sees something now and then tries to find a rationale for it," he says. "This is how history is produced in general. We see something now and we try and make a story that is cohesive to explain it. But that's not what I see. It is true that there are certain abilities that I had, and I was also fortunate to be in a western country with access to financial resources and the internet, and there are very few people who have the particular constellation of abilities and connections that I did. It is also true that I have always been interested in politics, geopolitics, and possibly secrecy to some degree." This is not really an answer, but it's all I'm going to get. Again like Warhol, there is an air of cultivated vagueness.

In his talk, Assange had said that he is neither of the right nor the left – his enemies are forever trying to pin labels on him in order to undermine his organisation. What matters first and foremost is getting the information out. "First the facts, ma'am," is how he summarises his philosophy to me. "Then we'll get down to what we want to do about it. You can't do anything sensible until you know what the situation is that you're in." But while he rejects political labels, he says WikiLeaks does have its own ethical code. "We have values. I am an information activist. You get the information out to the people. We believe a richer intellectual and historical record that is fuller and more accurate is in itself intrinsically good, and gives people the tools to make intelligent decisions." He says an explicit part of their purpose is to highlight human rights abuses, no matter where they are carried out or who perpetrates them.

He has described the provision of a safe platform for whistleblowers – his key tenet is the protection of sources – as a calling, and I ask him whether this will now always be the core of his life. His reply surprises me. "I have many other ideas, and as soon as WikiLeaks is strong enough to flourish without me I'll go on with these other ideas. It is strong enough to survive quite well without me now, but I don't know that it would flourish."

Is WikiLeaks's impact in the four years since it was founded an inherent criticism of conventional journalism? Have we been asleep on the job? "There has been an unconscionable failure to protect sources," he says. "It is those sources who take all the risks. I was at a journalism conference a few months ago, and there were posters up saying a thousand journalists had been killed since 1944. That's outrageous. How many policemen have been killed since 1944?"

I misunderstand him, thinking he is bemoaning so many journalistic deaths. His point, though, is the reverse – not how many journalists have been killed in the line of duty, but how few. "Only a thousand!" he says, his voice rising a little when he realises I haven't grasped his point. "How many have died in car accidents since 1944? Probably 40,000. Police officers, who have a serious role in stopping crimes, far more of them die. They take their job seriously." But journalists take their job seriously," I protest. "They don't take their job seriously," he says. "Nearly all of the thousand who've died since 1944 have been stringers in places like Iraq. Very few western journalists have died. I think it's an international disgrace that so few western journalists have been killed in the course of duty, or have been arrested in the course of duty. How many journalists were arrested last year in the United States, a country of 300 million people? How many journalists were arrested in the UK last year?"

Journalists, he says, let other people take the risks and then take the credit. They have been letting the state, big business, vested interests get away with it for too long, and a network of hackers and whistleblowers hunched over computers, making sense of complex data and with a mission to make it freely available, is now ready to do a better job. It's an incendiary argument, and one I'd stay and contest if he wasn't sipping white wine and about to order dinner. But one thing I would point out. The number of journalists killed since 1944 is closer to 2,000. After all, remember, accuracy, getting the facts straight, presenting the truth unvarnished, is everything in the brave new media world.