Patti Smith raises her arms and lets loose a howl. "What is the point?" she wails. "What is the point?" A wall of guitar noise ebbs and screeches, with the occasional crunch of feedback. She blows frantically into a clarinet, eliciting a succession of piercing squeaks, then spits on to the stage floor. We may be a well-heeled crowd at the South Bank in London, but she has conjured us into a smoke-filled poet-hole in Greenwich Village. About half of the audience look like they are on a spiritual journey; the other half are clearly keen to get home and have a stiff whisky.
She may be approaching 60, but Smith has lost none of the bohemian spirit for which she was so well known. Her live performances, during which she draws audiences into an improvised world of pure emotional energy, have been described as "religious", even "shamanic". With her unbrushed tresses of greying hair, outsized cowboy boots and bony, angular features, she is an icon whose name sits comfortably in the rock pantheon alongside the likes of Bob Dylan, Jimi Hendrix and Jim Morrison.
The odd thing is that she has not sold very many records. She had one top 20 hit, and that was in 1978 - a duet with Bruce Springsteen called "Because the Night". Smith herself recognises this: when she released Land 1975-2002, a collection of songs from her previous albums, she joked that it wasn't called Greatest Hits because "we would have had to call it Greatest Hit and have the same track 15 times over".
Nevertheless, she maintains an extraordinary influence over popular culture, continuing to play concerts all over the world. When she curated the Meltdown festival at the South Bank last year, it was the hottest ticket in town. It seems a strange paradox: Smith fulfils our desire for a true, pure artist - and yet she is clearly famous for something other than her art.
I put this to her when we meet, a few days before the concert, at the Alison Jacques Gallery in central London. She is launching an exhibition of portraits by Robert Mapplethorpe, her late best friend and sometime lover. In pride of place is the portrait of Smith from the cover of Horses: a familiar, androgynous figure in shirt and braces, staring confrontationally from the frame. This is the image that defined the rock'n'roll female, establishing a tradition that has been kept by everyone from Chrissie Hynde to P J Harvey.
In person, Smith is grounded and mellow. She sometimes takes a surprisingly homespun, motherly tone. "I would like to think that the quality of the work I do with my band merits people still being interested," she says. "Also I'm politically concerned, and I voice my concerns - perhaps that's another reason."
I wonder whether she would agree that people are fascinated by her as a symbol of a more dig nified age - a kind of anti-Paris Hilton. "I think there is a certain amount of truth in that. When I made my first record, rock'n'roll was a new form. I didn't think about making money, I didn't imagine being rich and famous. My motivations were not to get a bunch of cute guys, get drugs and have a limousine. I really wanted to do something important to contribute to the canon of rock'n'roll."
Smith's career has always been driven by a sense of mission. When she first started reading her poetry backed by an electric guitar, in 1970s New York, she wanted to "return rock'n'roll to the people". She was alarmed by the commercialisation of the music industry, as the gen eration of rebel artists of the 1960s either went stellar (the Rolling Stones) or bit the dust (Hendrix and Morrison).
"I came from an era when people felt they could make a difference. We felt that we could stop war. We felt that we could start a revolution, express our poetic and sexual energy, do something positive. That is part of the legacy of rock'n'roll and people can still do that. Some 16-year-old could wake up tomorrow and say, 'I'm going to make a record which will wake up the world.'"
Few artists who spoke for the children of the 1960s or 1970s would still talk so idealistically about the transformative potential of rock mus ic. And yet, unlike so many of her rock peers, Smith has studiously avoided the hedonism and self-importance of many people who work in the music industry. At the height of her fame in the 1970s, she suddenly retired to concentrate on raising a family with her husband, the guitarist Fred "Sonic" Smith.
"She is one of the few performers I know who has actually managed to get off the road," says Glenn Max, contemporary culture producer at the South Bank, who worked closely with Smith on last year's Meltdown. "No male artist I know has ever succeeded in doing that."
Predictably, it has been suggested that her husband demanded that she retire, but Smith herself has always insisted that it was her decision. "I had said everything I knew how to say, so I would either be redundant or merely start getting rich and famous," she told one interviewer when she re-emerged with the album Gone Again in 1996. "I needed to learn more, so I had more to give."
It has proved to be a wise choice. Family life has given Smith a solid foundation from which to continue her work, and a genuine sense of perspective on her fame. "I really don't want you to think that I sit around at home thinking of myself as an icon," she tells me at one point, with great emphasis. "I'm a mother, I'm a normal person, and I never think about myself in those terms, although of course it is flattering to hear it. I believe that fame is fleeting: it is only good work that endures."
Raising children has also given Smith an instinctive sympathy with younger people, who she believes are doing their best in a hostile world plagued by materialism and "devoid of spiritual content". All one can do, she says, is offer them positive alternatives. "You can't judge young people - you have to set an example. Give them tools. Try to inspire them. I know the new gen eration will figure it all out. And if I can answer any questions, if I can be of any help or service, then, you know, I'm around."
When I press her for detail, her counsel takes a somewhat surprising form: "Things like, eating a lot of fast food is really bad for you. All that salt and sugar creates high blood pressure; it makes you overweight, it makes you sluggish. And take care of your teeth, because when you get older it's such a drag. There's nothing worse than feeling creative, wanting to do stuff, and not being able to because you've got teeth problems and you don't have enough money to take care of them." Thus speaks the rock goddess.
While Smith's ability to hang on to her youthful idealism has endeared her to fans worldwide, it has a less cuddly side. She earned the anger of many American liberals for supporting Ralph Nader as Green Party candidate in the US presidential elections in 2000. Nader was blamed for George W Bush's victory when he won nearly 98,000 votes in Florida, where Al Gore lost to the Republicans by a slender margin. I ask her if, with hindsight, she ever regrets that decision. "Absolutely not. Ralph is a great man and if he was president the world would be glowing." She argues that Gore ran his campaign poorly, failing to fight back against corrupt tactics from the Bush camp. "A lot of people find it convenient to blame Ralph Nader. But the truth of the matter is that if Al Gore had fought to the end, the outcome would have been very different."
It is more difficult to justify her insistence that she would do the same again in the 2008 elections. "Would I campaign strategically to get rid of Bush? No. I would always back the candidate I believed in. If I'm voting for someone, I have to feel that I can live with myself about it."
It seems appropriate that our interview ends with that uncompromising statement. While those around her have resigned themselves to realpolitik, Patti Smith is still rooting for her dreams. And even if she sometimes sounds a wrong note, the world is a richer place for it.
"Robert Mapplethorpe - Still Moving & Lady" is at the Alison Jacques Gallery, London W1, until 7 October. www.alisonjacquesgallery.com
Patti Smith: her life and times
1946 Patricia Lee Smith is born in Chicago to an atheist father and devout Jehovah's Witness mother.
1967 Aged 20, she moves to New York and meets her lifelong friend Robert Mapplethorpe.
1974 She begins her musical career, initially with the guitarist and rock archivist Lenny Kaye, and later with a full band. Financed by Mapplethorpe, the band records a first single, "Piss Factory/Hey Joe".
1975 The Patti Smith Group is signed by Arista Records. Smith's first album, Horses, is released to great acclaim.
1977 While on tour, Smith accidentally dances off a high stage in Tampa, Florida, falling 15 feet into a concrete orchestra pit and breaking several vertebrae in her neck.
1979 Following the release of Wave, Smith meets and then marries Fred "Sonic" Smith, guitarist of the rock band MC5. She passes most of the 1980s in semi-retirement, living in a suburb of Detroit with her family.
1989 Robert Mapplethorpe dies of Aids.
1994 Fred, her husband, and Todd, her brother, die within months of each other. Two years later, Smith releases the album Gone Again and gets back on the road.