In an airless rehearsal studio, hidden away among the anonymous warehouses of New York's meat-packing district, Philip Glass and his ensemble are making final preparations for a rare performance of his 1974 landmark piece of pure minimalism, Music In 12 Parts. It takes six hours to perform, a work that pushes the boundaries of musical and mental stamina. It begins innocently enough, the opening of part one sounding like a chorus of angels, their needle stuck in some serene, celestial groove. But soon the arpeggios of Glass's Hammond organ break over the audience, releasing a torrent of shimmering octave notes and pounding triads. As the phrases repeat mercilessly the impression of heavy machinery at full throttle, cogs spinning furiously, pistons shunting back and forth, is hard to escape. Even in the stuffy rehearsal room, the impact of this bleak, mutating sonic landscape is alternately jarring, grating, mesmeric.

For the musicians - three other keyboards, soprano, tenor and alto saxes, flute and solo voice - the whole process is infinitely more exacting. The scores look less like conventional musical annotation than the peaks and troughs of a cardiograph. The only guide through the maze of dizzying circular melodies and perpetual rhythmic motions is Glass himself, who sits at his keyboard in the centre, periodically throwing his head forward in a manner not unlike a rock-guitarist launching into a power chord, the signal that nudges the music on from one whirling passage to the next.

During a break, Glass outlines some of the difficulties involved: "If we were to attempt this work," he says, "without preparing ourselves just the way a runner would, or a swimmer, or anybody who was involved with any demanding physical activity, we would be in trouble." At this point he is joined by the craggy, lanky figure of Michael Reisman, musical director and primary keyboard player of the ensemble. "Occasionally, I get not so much concentration lapses as fog - brain fog," says Reisman. "After the fourth hour, I sometimes get to the point where I start looking down at my hands and I can see them moving but I don't feel it's me any more."

Glass chuckles, and takes his place again behind the keyboard. "The trance aspect of the music is a misnomer," he says, releasing a flourish of arpeggios. "It's music that requires alertness all the time - if you space out, without any doubt you'll get lost. The audience might have that privilege, but we don't." In the quarter of a century since the conception of this unashamedly avant-garde work, Philip Glass has risen to become the world's most celebrated contemporary composer. Commanding fees in the range of £230,000, the sum which New York's Metropolitan opera house stumped up for The Voyage in 1992, he now has over a dozen full-length operas to his name.

While pioneers such as La Monte Young and Terry Riley may have first explored the form, it is Glass who is widely regarded as the founding father and figure-head of minimalism, one of the most influential movements of recent times. "Philip is probably the most successful living composer, and the one who has attracted the highest rate of commission fee since Verdi wrote Aida," says fellow minimalist Gavin Bryars. "His operas especially have been instrumental in opening up opera houses to a new and younger audience. His best operas - Satyagraha, Akhnaten, The Fall Of The House of Usher - are major works in the genre and stand out as some of the high points of late 20th-century opera."

On December 6, the BBC Symphony Orchestra will celebrate this achievement with a composer portrait of Glass at the Barbican in London to include the UK premieres of his second and third symphonies. December also sees the release on the Nonesuch label of a five-disc compilation of his sound-track work, entitled Philip on Film. However, as Music In 12 Parts graphically demonstrates, Glass's journey is all the more extraordinary for having begun on the fringes of downtown New York's non-commercial experimentalism.

The radicalism of early pieces, which also included the equally repetitive and scaled-down Music In Fifths, 1969, and Music With Changing Parts, 1970, enraged audiences and baffled critics. "Music of Philip Glass Called Sonic Torture," was the headline on one critic's view in 1973, which continued with "Music In Fifths sent a number of listeners racing out, their hands covering their ears. Through the performance, the audience continued to dwindle steadily."

The turning point came in 1976 with the landmark Einstein On The Beach, a startling five-hour collaboration with theatre director Robert Wilson, for which Glass supplied a scintillating score that wore its structure, rather like the Pompidou Centre, on the outside. "One of the seminal artworks of the century," raved the Washington Post, "possibly the seminal work." Glass cemented his reputation with the subsequent two instalments of his trilogy, Satyagraha, 1980 and Akhnaten, 1984. Then, having brought his work to the very boundaries of the definition of music, Glass proceeded in the 80s and 90s, to bring it all the way back again. "After stripping it down to a tiny subset," says Reisman, "he has gradually introduced one element after another to the point where the symphonic music he is writing today could have come from an entirely different place."

Some have been disappointed by the conventional nature of pieces such as Hydrogen Jukebox 1990, with a libretto by Allen Ginsberg; The Voyage, based on Columbus's discovery of America; and Glass's trilogy of operas based on Cocteau's films La Belle et La Bête, Orphée and Les Enfants Terribles. But, as others point out, the individual works are now dwarfed by Philip Glass the cultural phenomenon. "His success goes beyond classical music," says producer David Free man, who brought Akhnaten to the stage. "It is very hard to sit through an entire evening of television without coming across some sort of sub-Philip Glass rip-off music. He has become part of the musical lingua franca of our time."

Sitting in the claustrophobic back yard of his brownstone apartment in New York's East Village, Glass now bristles at the very mention of the term "minimalism", which he feels distorts conceptions of his music and traps it in the past. "People hear what they want to hear, see what they want to see," he grumbles. "If someone is told that a work is going to repeat over and over again they will hear it." But even he expresses a certain degree of wry surprise that the music has proved quite so versatile and enduring. "When it first came out, people said 'where can this music go?"' he says, betraying the flicker of a grin. "Well, it turns out it can go everywhere."

In conversation, Glass is naturally understated, deadpan, droll, with a distracted professorial air. People generally take to him. "What really impressed me is that he is so humble by nature," says Ravi Shankar who worked with him on the record Passages. "He is very simple, all these are things that are very rare in western people, particularly in famous ones." Glass has had his fair share of incendiary confrontations, the most famous being with Robert Wilson after Einstein On The Beach, when exhaustion, a shared $150,000 debt and what one friend has described as Glass's legendary canniness with money contributed to an icy silence that did not crack for years.

And in relation to his own work, Glass is anything but easy-going. "This is one of the most disciplined guys you will ever meet," says Godfrey Reggio, director of the Qatsi film trilogy, for which Glass has provided the sound-track. "He has literally 100 things going at once and all with the same careful attention."

Whether in New York, or at the fisherman's lodge in Nova Scotia which he bought with his first wife, the theatre director JoAnne Akalaitis, Glass rises at 6am, and launches into an iron-clad regime of work that only accommodates a modest break in the afternoon to deal with logistical matters. "The perfect timetable for Philip is when he can step off the plane and straight out on stage, with absolutely no time wasted," says Jim Woodard, tour manager for the ensemble. "Sometimes, I'll get him to the venue 20 minutes early, and he'll look at me as if to say: 'What am I supposed to do now? What am I doing here? I could have written a page of music in this time.'"

Earlier this year, Glass married Holly Critchlow, a restaurant manager and businesswoman some 30 years his junior. This relationship followed a particularly difficult period in Glass's life after his third wife, the artist Candy Jernigan, died unexpectedly of liver cancer in 1991 aged 39. "It was diagnosed very late," he says. "She found out six or seven weeks before she died. She was going to live forever, as far as I was concerned. It was a big shock for everybody, particularly the kids. Especially for people her age it was a big surprise." Jernigan's bold, pop-influenced paintings, bearing titles such as All Kinds Of Dope and Goya Beans still decorate his walls.

A Buddhist and vegetarian (and a vegan until his friend Doris Lessing convinced him of the dangers of dwindling bone density), Glass has never been more productive or content. He says "I'm a highly socialised person. I had a few wives, I have a number of kids and I have lots of friends. I have an ensemble, but besides that I have a recording studio and a production company. It is a bit of a nuisance to fit everything in, since I also need hours and hours to write music."

Philip Glass was born on January 31 1937 in Baltimore, Maryland, to Jewish immigrant parents, Benjamin Charles Glass and Ida, née Gouline. His father was a radio repair man who decided to diversify by stocking records. "Very quickly, I guess, the records took over the store," Glass recalls, "but he still kept a little place at the back where he fixed radios, because he liked to do it." It was here that young Philip was first exposed to a staggering variety of music, from bebop to hillbilly as well as more offbeat works by composers like Bartok and Hindemith, which his father brought home. "He told me later that he listened to these records to see what was wrong with them. He wanted to figure out why they didn't sell. That was his motivation. Because he was a practical guy."

Glass has an older sister, Sheppie, who has worked for humanitarian organisations, and a younger brother Martin, a businessman who lives in Baltimore. At the age of eight Philip became the youngest-ever student at Baltimore's prestigious Peabody Institute, taking lessons in violin and flute, and by his mid-teens was already composing. Then at 15, he took up a place at the University of Chicago to study mathematics and philosophy.

Soon, however, he found himself gravitating towards the music department, where he discovered the work of Charles Ives, Aaron Copland, Roy Harris and William Schuman. He also took piano lessons from fellow student, Marcus Raskin. "One day I was in the lounge playing the piano," says Raskin, who is now head of the Washington Institute for Policy Studies, "and he came up and he said: 'You know, I would like very much to learn to play - and can we begin with Bach? So, that is what we did; with two-part inventions." Raskin remembers a focused, eloquent and assertive young man, who even at such a young age seemed to have an idea of where he was going. He says, "He knew what he wanted to do and was prepared to bear the cost." After college, Glass went back to Baltimore and spent six months working as a crane operator at the Bethlehem steel mills to earn money for his postgraduate studies.

In 1957, aged 20, he arrived in New York to take up a place on the composition course at the Juilliard School, where students included that other minimalist firebrand Steve Reich and the teaching staff was led by Vincent Persichetti and William Bergsma. After experiments in Chicago with 12-tone serialism, it was here that Glass began composing in earnest, producing about 75 pieces over the five years of the course: "His music then was quite lyrical," remembers fellow student Peter Schickele, better known as the composer PDQ Bach. "Someone like Samuel Barber came to mind. There was a kind of dissonance between Philip's music and his tastes in cinema or literature, which were much darker; Ingmar Bergman and Céline were his chief passions."

It was also in New York that Glass cemented his relationship with JoAnne Akalaitis, then an aspiring actress he had met during his time in Chicago and would go on to marry in 1965. The y had two children, Zachary, born in 1969, now a budding songwriter, and Juliet, born 1971, a professor of history. Akalaitas says "I remember he invited me to go on a motorcycle to Aspen, Colorado. Phil at that time was a great biker. He loved road bikes and had this great big white BMW. It was very beautiful." After graduating from Juilliard, Glass spent two years on a composer in residence scheme in the Pittsburgh public school system.

Then in 1964, with Akalaitis following shortly afterwards, he sailed for Paris on the Queen Mary to study with the legendary music teacher Nadia Boulanger: "I was working six or eight hours a day just filling up pages and pages of music paper," Glass recalls. "The worst time was the 12 o'clock lesson. Since Boulanger never stopped to eat, her lunch was served to her at - or rather on - the piano. My attention was torn between the very real danger of food sliding onto our laps and the much more serious danger of her discovering that 'hidden fifths' had somehow slipped into my music."

However, if Boulanger gave Glass the technical framework for his radical new direction, the catalyst was an encounter with Indian sitar player Ravi Shankar, who was working on the sound-track to the Conrad Rook film Chappaqua. Given the job of transcribing the music to western notation, Glass first discovered the Indian use of rhythm "in developing an overall structure in music". In reaction to the infinite variations of the European serialists, who, Glass now believed, "wielded much more power than their appeal justified" and inspired by much of the progressive theatre work he had seen in Paris, including Beckett's and Brecht's, Glass now used these twin influences to pare down music to its reductive, basic elements.

Fittingly, it was in Akalaitis's own Paris theatre company, an ad-hoc group of ex-pat friends including Lee Breuer, Ruth Maleczech and Terry O'Reilly but which would soon become known as the seminal avant-garde group Mabou Mines, that Glass's new minimalist direction was first unveiled. "I think I was the first to play Philip's new departure," says saxophonist Jack Kripl, who played Glass's music for a production of Beckett's Play. "It was two notes, some sort of minor interval, played over and over. It was very enticing, and very technically demanding, which was I suppose a way of keeping that tension. These little intervals seemed to be a very powerful way of controlling the atmosphere."

After a grand backpacking tour through Turkey, Iran, Afghanistan, Pakistan, and India, Glass and Akalaitis returned to New York early in 1967. Within a few years their marriage had began to unravel. "I can hardly remember why," says Glass, "No-one threw anything at anybody. I think she was glad to get rid of me." They finally divorced in 1980, and Glass married Luba Burtyk, a doctor.

Back in New York, Glass renounced his earlier work and began to explore the possibilities of his new minimalist direction, with pieces such as Music In Fifths, Music In Similar Motion, both also from 1969, which were little more than single, haunting phrases played ad infinitum and at deafening volume. However, it was only with Music With Changing Parts, 1970, a swirling miasma of pulsating arpeggios that Glass began to fully develop his theory of "additive process" in which repetitions of musical phrases mutate slowly by the periodic addition of extra notes, and "cyclical structure", the process whereby two or more different rhythmic patterns played simultaneously arrive naturally back at a point of synchronicity.

The purest and most uncompromising realisation of these theories came with Music In 12 Parts, originally intended as a short exploration of 12 lines of counterpoint, but which grew into a mammoth study in rarefied, hypnotic minimalism. "I played it for a friend of mine," says Glass, "and when it was through she said, 'That's very beautiful; what are the other 11 parts going to be like?' I decided to take it as a challenge and compose 11 more parts."

Critical reactions to these pieces was often extremely negative. "It is as if Beethoven had made his entire career rewriting the first movement of the Moonlight Sonata," complained one critic. In 1973, a local Cleveland paper complained: "There was admittedly a certain masochistic fascination in the process. And when the music finally stopped, there was the satisfying feeling of relief that came from having suffered and survived."

Throughout this period, Glass supported himself as a New York cabbie and as a plumber, occupations that often led to unusual encounters. "I had gone to install a dishwasher in a loft in SoHo," he says. "While working, I suddenly heard a noise and looked up to find Robert Hughes, the art critic of Time magazine, staring at me in disbelief. 'But you're Philip Glass! What are you doing here?' It was obvious that I was installing his dishwasher and I told him I would soon be finished. 'But you are an artist,' he protested. I explained that I was an artist but that I was sometimes a plumber as well and that he should go away and let me finish."

The project that would propel Glass from downtown New York notoriety to international stardom came in 1976 - an avant-garde music theatre collaboration with theatre visionary Robert Wilson. Glass had his first encounter with Wilson's work at a 1974 performance of the glacial, imagist 1974 piece The Life And Times Of Joseph Stalin, and they immediately hit it off. Wilson says, "We readily understood one another because we thought alike. The way he was making music and how I was structuring theatre, it was very similar. We had a common sense of time."

Initial suggestions for subject matter included Hitler and Gandhi, but once they settled on Einstein, the piece began to evolve. "We met once a week in a little restaurant," says Glass, "and over six or eight months Bob produced a book of drawings that became the basis of Einstein and in turn became the basis of the music." Wilson's non-narrative, pictorial libretto, in which Einstein's theories and aspects of his biography were transformed into huge space ships and ghostly trains, and Glass's incantatory score, with its solfège arias [sung to the same syllable or syllables] and soaring streams of arpeggios has been cited as the perfect fusion of sound and image, though the two rarely discussed each other's contribution. "The only comment that Bob ever made was in the final rehearsal," says Glass. "I was using a piccolo for a certain section and he said: 'Isn't that a little shrill?' I changed it to a flute."

Einstein on the Beach was premiered in Avignon on July 25 1976. Glass and Wilson were then offered the option of two performances at the Metropolitan Opera in New York, where the critical reaction was delirious: "One listens to the music just as one watches Wilson's shifting tableaus," wrote John Rockwell in the New York Times, "and somehow, without knowing it, one crosses the line from being puzzled or irritated to being absolutely bewitched." The day after the performance, Glass was back driving his taxi: "I vividly remember the moment, shortly after the Met adventure," he says, "when a well-dressed woman got into my cab. After noting the name of the driver, she leaned forward and said: 'Young man, do you realise you have the same name as a very famous composer'."

Einstein had, however, changed everything. During the tour, Glass was approached by Hans de Roo, head of the Netherlands Opera. "Well, Philip, that was very interesting," he said, alluding to the fact that the piece was performed by Glass's ensemble and sung by semi-professionals. "Now, how would you like to write a 'real' opera?" Glass immediately decided on subject matter that had been rejected by Wilson during the process of formulating Einstein: Mahatma Gandhi's formative period working as a lawyer in South Africa. Research on the piece that became Satyagraha began with a trip to India, in the company of librettist Constance De Jong. Slowly, a libretto was created that combined elements of Gandhi's life with extracts from the Bhagavad-Gita. "We really shepherded the work from my kitchen table to the premiere," says De Jong. "It was never outside our hands for very long."

However, a "real" opera brought new challenges, including a full quota of divas who fiercely resisted the unorthodox choice of Sanskrit as the predominant language of the libretto. "A lot of the singers were appalled by the idea," says De Jong, who was given the job of teaching the dead language phonetically to the chorus. However, most mutinous were the musicians, who objected to the continuous repetitions. "Oh my goodness they were horrible," moans Glass. "At one point, they just put down their instruments and walked out." However, no-one could argue with the overwhelmingly positive critical reaction to Glass's lush, beguiling score when the opera was premiered in Amsterdam in 1981. New York Times reviewer Robert Jones, who had described Glass's early works as "lacking even the sophistication to raise them into the class of the primitive" was now enthusing that Satyagraha "shines with a luminous beauty that makes it the most sensually appealing of all Glass's works".

A commission of the third instalment of the trilogy came in 1982 from the Stuttgart Opera. Glass soon settled on the subject matter of the 18th dynasty Egyptian Pharaoh Akhnaten, credited as the founder of monotheism. Again the singers balked at the prospect of tackling a libretto assembled from fragments of the Babylonian language of Akkadian, ancient Hebrew and ancient Egyptian. However, the orchestration is far more conventional in tone and colour than its predecessors, signalling a move away from avant-garde experimentation. "Einstein remains not only among my greatest achievements but I don't know any contemporary opera which has challenged the form as much," Glass says. "But I could see no point in writing Son Of Einstein."

Since completing the trilogy, Glass's output has been impressive. He has written over a dozen new operas, including The Making Of The Representative of Planet 8 (1988), with a libretto by Doris Lessing; The Voyage (1992) and 1000 Airplanes On The Roof (1988), with libretti by David Henry Hwang; and collaborations with Wilson, including a 3-D opera Monsters Of Grace (1998), and White Raven, seen earlier this year at New York's Lincoln Centre.

Some have lamented that the earlier, ascetic rigour of the work has disappeared. But others feel it has simply grown richer and more mature: "Before, with that minimal thing he did, it was very much a type or genre of music," says Shankar, "but this past 10 years I find he has grown so much and there are so many different aspects. His creative range has become so much larger."

The last decade has also seen upheavals in Glass's personal life, in particular Candy Jernigen's death in 1991. However, five years ago Glass met Holly Critchlow, now his fourth wife. "The age difference doesn't seem to be a hurdle. But that is partly because I've always had younger friends and collaborators," he says, adding "I've outlived one wife. So I am not so sanguine as to think that I won't outlive this one. One thing that you find out is that you have no way of knowing. It is completely unpredictable."

However, from Music In 12 Parts through Einstein all the way to the 6th symphony on which he is currently working, one aspect of Glass's life has remained constant: "I am still trying to write melodies which are truly beautiful and fresh and unexpected. It's very simple: I find playing music and writing music very challenging. It hasn't gotten any easier as I've gotten older. And after 50 years of composing, that is an achievement in itself. It is still engrossing enough to get me up early and keep me working all day."

Life at a glance: Philip Glass

Born: January 31 1937, Baltimore, Maryland.

Education: Peabody Institute 1945-51, University of Chicago '52-6, The Juilliard School, '57-'62, Nadia Boulanger, '62-4.

Married: JoAnne Akalaitis 1965 (one son Zachary, one daughter Juliet) dissolved '80; Luba Burtyk '80 dissolved; Candy Jernigan (died '91); Holly Critchlow 2001-.

Works Include: Music for Play 1965; Strung Out '67; Two Pages '68; Music In Fifths, Music In Similar Motion '69; Music With Changing Parts '70; Music In 12 Parts '74; Einstein On The Beach '75; Satyagraha '80; Koyaanisqatsi '82; Akhnaten '84; The Juniper Tree '84; The Making Of The Representative For Planet 8 '86; 1000 Airplanes On The Roof, The Fall Of The House Of Usher '88; The Screens, Hydrogen Jukebox '90; White Raven '91; The Voyage '92; Orphée '93; Monsters Of Grace '98; Dracula '99.