Arianna Huffington sweeps into the room at the Time-Life building in midtown Manhattan. She is tall and statuesque and waves a mane of chestnut hair above cheekbones so sharp you could hang a jacket on them. She does not look around. She does not need to. Everyone is looking at her.

Hurricane Arianna is hitting New York. She blows through the room, meeting and greeting the powerful Manhattanites gathered there to debate who should be Time's Person of the Year. It has probably occurred to a few of them that it just might be her.

At the party she is a social whirlwind, but she is also the harbinger of a very different sort of storm. For Arianna has become the unlikely face of the internet revolution that is sweeping through the world's media, tearing down the walls of old-media fortresses (including Time) like a geeky mob of nerds assaulting the Bastille. Her current affairs blog, the Huffington Post, just 18 months old ("delivering news and opinion since May 9, 2005", it boasts on its home page), is now one of the world's most influential media outlets. "We hold the mainstream media's feet to the fire," she says, smiling the certain smile of the true believer.

To watch Arianna at work is to see a human blog in action. Each air kiss seems like the click of a computer's mouse, each handshake a link to another potential blogger in an ever-growing network of movers and shakers. No wonder she has prospered in the world of the web. At the Time lunch she meets Hollywood star Emilio Estevez. Click. Later, at the Council on Foreign Relations, the glamorous Hungarian-American chess grandmaster Susan Polgar eagerly presses her business card into her hand. Click. By evening she is at CNN headquarters where top anchor Paula Zahn greets her with a girlish exclamation: "Arianna!" Click.

Every moment of downtime, while she is being made up for an interview or driven to a meeting, is spent in online communication. Arianna is permanently clamped to her BlackBerry. It is this high-octane power networking that has allowed the Post to gather the greatest roster of celebrity names in the blogosphere, from media stars such as Tina Brown and Norman Mailer to Hollywood figures like John Cusack, Tim Robbins, Steve Martin, Alec Baldwin and Larry David. There are also politicians such as John Kerr and Gary Hart and, amazingly, not one of these contributors gets paid.

Here and now, Arianna is in the inner sanctum of the enemy: Time magazine is the perfect symbol of much of the world's old media. And Arianna is having a ball. She even holds out an olive branch to the many newspapermen and magazine writers in the room, kindly declaring that she still reads them - "At least five newspapers a day," she says. "The argument that the old media will simply die off is becoming obsolete. Honestly, there is room for both of us. Both of us are here to stay."

But the very fact that Arianna feels she needs to make such a statement tells you everything about how far the revolution has already come. She is seeking to reassure the great and good of Manhattan's old media world that they still have a future. No wonder the photographers circling the event like vultures appear to swoop down only on her. "She's a true force of nature," says Richard Stengel, managing editor of Time.

The Post is now the fifth most popular site in the world. It shapes the debate of American politics and gives Arianna real power and prestige. This year she made Time magazine's list of the 100 most important people in the world, and next year she looks likely to climb the list.

Nothing happens these days that does not happen on the web. In Britain, America and across the world newspapers face declining sales and falling revenue as the advertising industry moves online. Journalists are being laid off everywhere. All Britain's major newspapers, from the Guardian to the Times, are ploughing huge resources into their websites, even as they scratch their heads wondering how to make them profitable. The whole landscape is shifting. There are now 115 million MySpace members. Google paid an astonishing $1.65bn for YouTube. As the economy changes beyond recognition, these companies are shaping up to become the new Wal-Mart and General Motors.

Politics is changing, too. America's midterm elections were largely defined by the internet, blogging and YouTube. Just ask the Virginia Republican ex-senator George Allen. He offhandedly called one Indian American political activist a "macaca" (a racist slur) - then found a video of the remark posted on YouTube and highlighted by blogs like the Post. It triggered the collapse of his campaign and the destruction of a political career many Republicans believed was headed to the White House.

Arianna rejoices in the changes to the political and media landscape. "The power of the internet allows us to influence what is happening. We can advocate for change now. Everyone can. The internet has democratised power," she says. Her next comment is delivered with a twinkle in her eye. "Don't worry. We at the Huffington Post have no plans for global domination," she says. The "not yet" was left unspoken.

Huffington is America's newest household name. The Post has already broken major news stories, changed perceptions and challenged the old way of doing things. She is a media magnate for a new age and uses her position to hammer away for liberal causes: the Iraq war, environmentalism, corporate greed.

At the Time meeting she is involved in a public spat with Brian Williams, news anchor of NBC, an old-media icon who represents a world where the family sat down in the evening to have the news read to them by a man only half-jokingly nicknamed the Voice of God. Huffington is the opposite. He despairingly berates the new world of blogging, YouTube and Google. "It could destroy us. We are choosing cat-juggling videos over well-researched newscasts," he says. "It is de-Americanising us." Arianna rises to the bait. "I am not sure what universe Brian inhabits, but it is not mine," she says. Later, she is still simmering. "I was amazed at what Brian said. Just amazed that he could say such things about blogging. It was ridiculous." Then she laughs: "After all, he blogs, too."

The Huffington Post offices are in a converted warehouse deep in the heart of New York's trendy SoHo. The news editor, Katharine Zaleski, is not shy about where she thinks the Huffington Post is going. "We would like to be the future of news," she says simply, Her tone suggests that it is only a matter of time. The Huffington Post's mix of comment, news and personal blogging gets 3 million unique visitors a month and more than 30m page views. These figures are growing all the time. The site is a must-see for every journalist and politician in America.

The real secret of the Post's success is not its star names but its army of citizen bloggers (including Arianna herself) who write freewheeling posts on a variety of subjects. They are men like Bob Cesca, a Philadelphia-based animator. "We can write whatever we want. There's no real censorship. This is raw, unfiltered power," Cesca says. Though there are conservative commentators, the tone is overwhelmingly liberal and it has already claimed some notable scalps. It was the Post that drove the Judy Miller scandal at the New York Times, panning her pre-Iraq war coverage of Saddam's non-existent weapons of mass destruction. As the Post began covering Miller's reporting, she was seen as a brave woman going to jail to defend her right to keep her sources secret. By the end of it, she was portrayed as a shill of the White House who had unquestioningly accepted the case for invading Iraq and acted as a spur for the war.

Whenever a big story breaks, the bloggers of the Post run with it, shaping the mainstream media's perception before they have published a word on paper. The Post's power stems from the secret of all blogging's success: increasingly it is interpretation, not facts, that matters.

Arianna Stassinopoulos was born in Athens on July 15, 1950. Her mother, Elli, who Arianna says has been the "greatest influence" on her life, left her unfaithful father partly at the young Arianna's instigation. "When she left my dad because of his massive philandering, she had no money, no job, no formal education. She was fearless. That was the kind of woman she was," Arianna tells me.

Her mother's inspiration gave Arianna the courage to apply to Cambridge University after seeing a picture of it in a magazine. It paid off. Arianna arrived in the UK burdened with an accent people still mistake for Hungarian. She left as head of the Cambridge Union, and a society beauty.

But there was one thing missing: the great love of her life, English journalist and writer Bernard Levin, would not marry her. She had had enough of Britain, though she remembers her years here as formative: "They were incredibly important times. I got my education in speaking out at Cambridge, at the Union. It is where I discovered both my heart and my mind. I learned how to argue. I apply that now in my blog," she says.

So Arianna launched her first great reinvention. Aged 30, she moved to America, landing in New York at the beginning of the city's gilded decade, the 80s. She conquered the city, and became Manhattan's premier It girl, appearing on the cover of New York magazine.

Then came marriage to Michael Huffington, an oil billionaire so reclusive only five people knew his phone number. Huffington had a modest political career in the Republican party, which gave Arianna a taste for power and conservatism. She signed up to the rightwing revolution headed by the Republican Speaker Newt Gingrich.

Then it all changed again. First, she divorced Huffington - who revealed he was gay. Second, she began to drift to the left, just as the Republicans began to veer to the right. "I was always a socially liberal Republican," she says now. "The party has changed so much now. But I also realised that corporate America was never going to step up to the plate and help society's disadvantaged by itself. Other people had to force it or do it themselves."

Her palatial home in California became a salon for every shade of liberal mover and shaker. From this base, Arianna tried to launch her own political career, campaigning against Arnold Schwarzenegger to become governor of California in the bizarre election of 2003. She dubbed the contest the Hybrid vs the Hummer. The Hummer flattened her. "It was tough," she says, "but I also learnt an enormous amount: the double standards about women and speaking out. But from that failure came a new venture. People should embrace failure. Women are so afraid to face failure."

And embrace it she did. From the ashes of political disaster was born the Huffington Post. Arianna, too, was reborn. In her latest and most powerful guise: internet media mogul.

Hurricane Arianna never stops blowing. This has led her detractors to sneer at her ambition. The jokes are legion and carry with them a whiff of snobbery and xenophobia. She has been called "the most upwardly mobile Greek since Icarus" and "the Sir Edmund Hillary of social climbers". For her public shifts of opinion she has been termed "an intellectual lapdancer".

Huffington is as ambitious as ever. Her constant criticism of the Iraq war has become an obsession. "It is a complete tragedy. We have thrown away so many resources that we could have used in other ways to really change the direction of the country and do some good. It is immoral.

It is unnecessary and the people who planned it cannot justify it," she says. Unlike many other leftist figures, Arianna's anti-war stance is no recent conversion. She backed anti-war mum Cindy Sheehan when few other liberals would go near her, offering Sheehan a blog which began, "This is George Bush's accountability moment. That's why I'm here. The mainstream media aren't holding him accountable."

In a liberal world dominated by Hillary Clinton such a sentiment is refreshingly direct And, unlike Hillary, Arianna has been speaking against Iraq since long before mainstream figures caught up with her. Nor is she just angry at the Bush White House. Her greatest ire is apparently reserved for Democrats, who she sees as having failed monumentally in coming to an anti-war position far too late. "The midterm elections was about three things: Iraq, Iraq and Iraq. What is galling to see is that it took so long for Democratic politicians to catch up to the American people. We did not have Democratic leadership," she says.

She also has little time for Tony Blair's decision to back the American-led war in Iraq so enthusiastically. "I think it is tragic what happened with Tony Blair. He was a man of tremendous gifts. I don't know how one can psychologically explain what it was that made him go along with this insanity in Iraq, despite all the mounting evidence against it."

Clearly Arianna is a woman who speaks her mind and her hyper-public presence makes her appeal to many women in an age where the female sex can still be preferred to be seen but not heard.

That is the theme of her latest (and 11th) book. It is a self-help guide titled On Becoming Fearless. It is a painfully personal work, detailing her struggles with insecurity, her daughter's anorexia and her own miscarriage. She says she wrote it for her two daughters, Christina and Isabella. "I saw in them a lot of the fears that I had dealt with, and I thought that we had overcome as a society and that our generation had vanquished. Yet they have to fight them all over again," she says.

She is not dating at the moment and seems happily single. "I love my life. Of course, I would like a man in my life, but I don't need one to feel fulfilled. A lot of women settle for the wrong person because they fear being alone. You are never alone when you do something you love."

At the Time party, as the official discussion comes to an end, there is a rush to the stage on which the panel sits. It is not Brian Williams people want to talk to most. Or Emilio Estevez. Or Tom DeLay. It is Arianna who is mobbed and fights her way slowly through the crowd smiling all the time and back into networking mode. The party is over, but it still takes her 30 minutes to leave the building. The human blog never stops making new connections.

The Observer