Philippe Petit went to New York for the first time in January 1974. The twin towers of the World Trade Center would be formally dedicated on 4 April: but even then they were not fully complete or occupied. When he sneaked into the north tower for the first time, the buildings were still under construction. He rode elevators and ran up staircases to evade security guards. It took him an hour to get to the roof. The next day he returned with his friend Jim Moore, a photographer, and took the same route to the 110th floor. Philippe explained what he had in mind. He showed Jim the drop. Jim just went white. 'You're insane,' he whispered.

Philippe returned to Paris and, ignoring the advice of his friends, began to make preparations for his third 'coup'. He read everything he could find about the towers and compiled a file of information. In April, he was back in New York again. He made money with his street juggling - nobody in Manhattan had ever seen anything like it - and began sneaking into the towers every day. He roamed through the buildings for hours, dodging guards, taking photographs and making sketches, noting access routes, security patrol routes, equipment clearances. He obtained numerical security codes; hired a surveyor's measuring wheel to determine the distance between the buildings; rented a helicopter to photograph them from above. He found that on windy days, the turbulence on the roof of the towers made it impossible even to stand up without holding on to something; and that the buildings swayed in a strong wind: enough to snap a steel cable tensioned between the towers. And he discovered the police station in the basement.

'Notre Dame and Sydney - that was nothing. Notre Dame doesn't have a police station, it is not 1,000 or so feet high. It was a public structure, very easy to access. And Sydney Harbour Bridge was half-and-half: a bridge, in the middle of the night. The World Trade Center was the end of the world. Electronic devices, police dogs. It was l'attaque de la banque. Bank robbery, you know?'

Back in Europe again, he persuaded his friend Francis Brunn to put up the money to help finance the 'coup'. He had a scale model of the Trade Center built: it filled his little room at rue Laplace. He sought detailed advice on rigging the wire and cavalettis from Papa Rudy. He decamped to his parents' country estate at Vary, and began practising in earnest. Finally, in May, he returned to New York one last time. He planned one date after another for the walk - and abandoned one attempt at the last minute - until fixing on 7 August 1974.

What did you think your chances of success were?

'Zero. Under zero. It was impossible. And the walk was not even to think of. I'm trying to sneak inside the biggest, most surveilled, protected building in the world. I was a kid from the street and I thought: maybe I could have two crews coming at more or less the same time and then putting a ton of equipment across and then guylining it and then tightening it - without being caught by all the cops and the guards? And you're asking me did I think about the walk? Of course not. The walk was a stupid, ridiculous objective. And maybe when I did think about the walk, it was nothing. I am a wire-walker. I can walk any time, anywhere - I'm indestructible. So the walk was never a subject. Really, the tough part was the bank robbery. Getting out alive? Pfft! I was not interested in that.'

As I approach the edge of the building, a bunch of arms reach out to assist me in taking the last step. Hey! I don't need help! I haven't finished my show! I come about. Behind me, the arms pull back. I rendezvous with the long wire and perform the 'torero walk', gliding my feet, holding the pole away from my body, head high.

Near the end of the crossing, another voracious group of arms tries to grab me. An octopus! Smiling at the monster, I stop short of its reach and make another U-turn.

Next crossing - I'm absolutely no longer afraid of the wire, it is getting shorter as I stroll back and forth. Next crossing, I present the 'promenade', balancing the pole on my shoulder like a pitchfork, one arm dangling, as if returning to the farmhouse after a long day's work in the field. Someone is calling me...

Each new crossing begets a new walk, punctuated by fragile equilibriums, by genuflections, by salutes. Again, I sit down; again, I lie down.

By chance, and for the first time, my eyes focus on the octopus at the end of the wire. It is made of several uniformed men. Those men are angry. How do you stop a wire-walker?

On the afternoon of 6 August 1974, Philippe and six accomplices set off for the World Trade Center in an unmarked van. With forged ID passes and dressed as delivery men, they had arranged one team to deliver all the packing cases of heavy equipment. The 60m cable, the machine to tension it and Philippe's 8m balancing pole disassembled into sections were to go to a friend working in an office on the 82nd floor of the south tower. But they arrived on moving day: several major corporations were moving into their new offices. So Philippe and his team managed to ride freight elevators all the way to the 104th floor. The inside man then escorted two others, disguised as businessmen, up the north tower. It was only 4.30 in the afternoon. Both teams had to remain hidden until nightfall before they could begin the rigging.

Over in Germany, at the Circus Sarrasani, Francis Brunn's wife Sasha became convinced Philippe was about to kill himself. She decided she must stop him. She called the police in New York to warn them to look out for strange activity on the roof of the Twin Towers. She tried World Trade Center security, and the Port Authority police. She called again and again. But she couldn't get through.

On the 110th floor of the south tower, Philippe and his friend Jean-François spent more than five hours motionless, concealed under a tarpaulin - sitting on an 8in wide I-beam over a three-storey drop. Off-duty construction workers had decided to hold an impromptu party on the roof. Then, still evading security guards, and with a line fired by bow-and-arrow by his friend Jean-Louis from the north tower, the two teams spent the whole night rigging. The first construction workers would arrive for work on the roof at 7am. By dawn, they still hadn't managed to tighten the cable. At 6.45, Philippe was still desperately trying to overcome difficulties with the cavalettis. One of his accomplices had simply given up helping. At the last possible moment, as the freight elevator was on its way up to the 104th floor with the first shift of workers, the Twin Towers of the World Trade Center were firmly linked for the first and only time in their existence. Philippe Petit finally began his walk.

Suddenly the shouting hurled at me reaches my ears, because this time, the words are in French. It is Jean-François, terrified by the threats of the police - they say they're going to loosen the tension on the wire, they say they're going to send a helicopter to snatch me from mid-air - who has agreed to translate their latest message: 'Stop right now or we'll take you out!' For a second I despise Jean-François, but then I understand: he believes them.

Fascinated by a sudden wave of silence, I retreat into an endless balance on one leg, searching for the immobility that no wire-walker can ever find on a wire. Today, maybe, with the help of such a prodigious height, I...

But I have trespassed long enough into these forbidden regions; the gods might lose patience. I offer my farewell to the New York sky: by running on the wire that shakes with allegresse, thus bringing down the curtain on the most splendid performance ever offered by a street-juggler/vagabond/high-wire artist.

'Look! He's dancing, he's running!' scream my friends below, applauding my exit. They argue over the number of crossings: 'He crossed six times!' 'No, eight!' 'He was on the wire 45 minutes!' 'No, an hour!'

I, a bird gliding back and forth between the canyon's rims, did not count the voyages. I land on the roof of the south tower. The octopus grabs me violently.

The World Trade Center walk immediately made Philippe Petit one of the most famous men in America. His picture appeared on the front page of newspapers across the country. The next day, Richard Nixon became the first president in US history to resign from office. Before leaving Washington by helicopter, Nixon met the press on the White House lawn for one last time. 'I wish,' he said, 'I had the publicity that Frenchman had.'

The phone rang with one offer after another. There was a multimillion-dollar film deal from MGM. There was a book proposal: How To Walk The Wire in Your Back Yard in Five Days. There were beer endorsements; wine commercials; Burger King offered him $100,000 to dress up as a Whopper and wire-walk across 8th Avenue to open a new franchise. Someone even wanted him to make a record, based on what he had said to the press after his arrest: 'When I see two oranges, I juggle; when I see two towers, I walk.' He turned them all down.

'I could have become a millionaire within days. A stupid book would have been made, a stupid film, stupid T-shirts, stupid little dolls climbing on the tower like King Kong would have been made. I didn't say no in principle, I said no because I looked at the people and heard their words and everything was wrong. There was a different language, it was a different point of view. It was not me. And I cannot be not me. I'm not honourable and courageous in my way of seeing life - I just cannot help it.'

But he did accept a permanent pass to the Observation Deck of the World Trade Center and, finally, joined the circus - as the headline act in the Ringling Brothers' original Greatest Show on Earth. It did not go well. He still didn't fit in to the world of the circus. He had his first and only fall - during a practice walk, from 45ft up. He broke several ribs and suffered internal injuries. But as he was rehearsing, he insists it doesn't count.

'It was practice. And practice is a very different field. Wire-walking in performance is one thing - I never fell, of course. If I had, I wouldn't be here talking about it.'

He settled in New York, and gathered a constellation of famous friends and admirers - Werner Herzog, Paul Auster, Mikhail Baryshnikov, Milos Forman, Robin Williams, Al Pacino, Sting. The actress Debra Winger once told him she has had it written into the contract of every film she has made that she must include a visual tribute to Petit somewhere in her performance. He's written several books about his exploits, and a treatise on wire-walking based on the notes he made as a teenager: Funambule. In 1980, he was made artist-in-residence at the Cathedral of St John the Divine. He has had rooms, and a practice wire, there, ever since.

Up in the little room on the rue Laplace, Philippe and I stand in our places - there is no room to move - and he shows me around. He proudly points out objects here, space-saving innovations there, the meticulously filed boxes of his writings; obsessively filed notes on past walks and future projects. He explains how he illegally installed running water and electricity. How he and Jean-Louis stole the heavy ceramic sink from a factory back in 1968. He tells me how, a few years ago, he excavated a recess in the ancient wall for a tiny fridge. He taped pieces of foam over his mallet and chisel, to muffle the blows, to prevent his neighbours being alerted. He listened at the wall with a stethoscope to find when they were out. It took him months. 'A grain of sand at a time,' he says. 'I am a specialist in a grain of sand at a time.' When he had finished, he found that working in the cramped space under the sink for so long meant that he could no longer stand properly. He couldn't walk the wire for a year.

This is the first time Philippe Petit has returned to Paris since 2000. It's his greatest absence for a long time. He says he just hasn't been able to afford the air fare. He has brought packets of soup with him from New York so he wouldn't be tempted to eat in restaurants every night. Lately, he hasn't worked as much as he would like. Some years, he's done six walks in a year. In others, none at all. He can only do a maximum of two or three 'big' walks a year, and they take a lot of preparation. 'It costs money. And it's complex. It's not in two days, you know. So it's easier to hire Marcel Marceau. He comes, he puts on his white, and he does his thing for 20 minutes. So it's a pain in the ass, a wire-walker. But it's an art that's rarely seen, the way I do it is unique, it has some value.'

When he does work, he earns a lot of money. But he isn't good with it. Mostly, it goes on paying back the friends who support him when he has none. 'I'm very good at spending money,' he says. Some of it, he spends on wine. There are a dozen bottles here, caked in years of dust - including a 1961 Bordeaux ('The best year of the century') and a £2,000 bottle of Château d'Yquem. 'But somehow I'm very bad at growing money. And I don't care... I know how to live with nothing.' And later: 'It's the most impossible profession in the world. I am the most stupid man, to have become a wire-walker. But again, I didn't choose it. It chose me. So here I am - a prisoner of something I love.'

When he couldn't walk, he used to do street juggling to raise money. But he hasn't done it in a while. The cities have changed, he says. People don't seem to have the time any more. They live in their own worlds of Walkmans, TV and the internet.

At his house near Woodstock, he has spent the past 10 years building a barn, using only 18th-century tools. And he worked on his book about his World Trade Center walk, To Reach The Clouds, for years - he interviewed all his friends, kept fastidious notes. He doesn't have a television, so on 11 September he had a phone call from a friend up the hill: his towers were being destroyed. He rushed to a neighbour's house and watched it happen, live.

Philippe Petit says he is never afraid when he is actually on the wire. But he always tastes a tang of fear around it. And often, after he has completed a walk, when he is on the ground, with the crew and the mayor and a glass of champagne, he will look up and be filled with terror by what he has just done.

At 53, he says he is stronger on the wire than ever. 'It's so easy to walk on the wire. I will stop walking when I cannot walk on the earth. If I have a cane and I barely can walk on the earth.'

He had hoped that the publication of To Reach The Clouds in America would help him find a backer for the greatest work so far, the Canyon Walk: 'Canyon Walk is a masterpiece for me. And Canyon Walk is an immense, worldwide event like walking on the moon for the first time. It will make me so well known all over the world that my life would completely change.' But so far, it hasn't. Instead, he's developed a script for a film. He knows exactly how it will open.

He and his old friend Francis Brunn, who turned 80 last year, will be sitting together on the Observation Deck of the World Trade Center. They will look out into the void: the unfathomable void. And Francis will tell Philippe he can hardly believe that he ever did the walk all those years ago. 'You know?' Philippe will say. 'I cannot believe it either.'

· Quotes extracted from To Reach The Clouds by Philippe Petit (£12.99, Faber and Faber)