Dressed in a flesh-coloured body stocking and long blonde wig, Sinead King shouts into a loud-hailer at Oxford Circus: "Anyone want a new hymen?" Her colleague, Katie O'Brien, pretends to inject her face with a giant syringe labelled "Botox". Every few minutes the pair down tools, belt up their red and white trenchcoats, put their hands in their pockets and "flash" passers-by, revealing curly merkins (pubic wigs). Onlookers laugh – and stare. Welcome to the world of the Muffia.

King, 28, and O'Brien, 27, met at a performance art workshop in Glasgow a year ago and started doing impromptu monthly street performances in Manchester and London last summer. Both have backgrounds in the arts: O'Brien did a fine art degree and works as a theatre administrator; King studied performing arts and has worked as a gender studies lecturer. But they specifically wanted to make a political statement. "We realised we were both quite radical," says King, "and that we didn't really want to define ourselves as artists. I felt that a lot of art lacks a political message. People were not asking the questions I wanted them to."

Under the guise of the Muffia, they started asking questions of their own. Why don't we resent the way the media portrays women? Does no one care that women are mutilating themselves with cosmetic surgery? Why do so few young women know what feminism is?

They performed their first public "gig" at London's Oxford Circus last May, posing as "fake FHM girls" with blonde hair, perfect makeup – and concealed merkins and voluminous fake underarm hair. When passers-by stopped to look, "we would flash our hairy muffs. Generally we got a smile or a laugh – unless we showed off our armpits, which mostly caused disgust." They have also dressed up as builders, wolf-whistling, "scratching our imaginary balls" and heckling men on a building site: "They loved it."

Dressed as the Muffia, they usually stand in the street writing messages on each other's body stockings, such as "Lose a few pounds" and "Eating disorders or a society of disordered eating?" O'Brien explains that their aim is "to use our bodies on the street to generate ideas and engage with people." To this end, O'Brien also once staged a one-woman performance outside Topshop, mixing up a pink concoction and then vomiting it into a bucket, to protest against the prominence of ultra-thin models.

Their main audience seems to be young women, aged 13 to 18, who have heard nothing about feminism. On the chilly Saturday I spend with them, groups of girls gather to watch the performance, giggling and frowning, running up to take pictures and ask questions. Shalini, 17, a student from north-west London, says: "They're so interesting. They're talking about how the media is making us have an image in our heads of how we should look. No one has ever talked to me about these things before. It's interesting because it made me think about how I feel about not being pale and blonde."

King and O'Brien are the latest in a long line of feminist performance artists, stretching back to the 1970s. Many of these women have created work that involves intentionally behaving badly in public – bucking notions of appropriate female behaviour – or exploring the idea of femininity as a performance. As Los Angeles feminist artist Cheri Gaulke puts it: "Performance is not a difficult concept to us [women]. We're on stage every moment of our lives, acting like women."

Feminist performance art also tends to revolve around the artist's appearance. In her piece Carving: A Traditional Sculpture (1972), Eleanor Antin documented her body over a 37-day period while she was on a crash diet, taking four photographs a day. This created a vivid portrait of a body deteriorating. For her Catalysis performances in the early 1970s, New York artist Adrian Piper travelled around the city, first in stinking, rancid clothes and later with a towel stuffed into her mouth. And around the same time, the Canadian video artist Lisa Steele videoed herself naked on her 27th birthday for Birthday Suit with Scars and Defects, pointing out all the flaws on her body.

In the 1980s, the focus moved from the individual to the culture at large, as heralded by the arrival of the Guerrilla Girls in 1985, a group who donned gorilla masks to protest at male dominance of the art world. Adopting the names of dead female artists (including Frida Kahlo, Lee Krasner and Eva Hesse), the Guerrilla Girls used posters and humour to get their message across. In 1989 their most famous billboard campaign used the slogan, "Do women have to be naked to get into the Met Museum?" following their infamous "weenie" count at New York's Metropolitan Museum, which had revealed that 85% of the nudes on show were female – while only 5% of the exhibited work was by women.

"The heyday for performance art was in the 1970s," says Dr Angela Dimitrakaki, lecturer in history of art at the University of Edinburgh, "but it had its roots in the 1960s hippy culture of sexual liberation. One of the most exceptional performances in my view was Carolee Schneemann's Interior Scroll (1975), where she is pulling a scroll from her vagina on which she has written a text about creativity. The whole movement is about how to construct from scratch a certain presence of the female body that does not capitulate to the patriarchy – and, specifically, to the capitalist patriarchy."

With its anti-consumerist messages, the Muffia is a part of this tradition. "It's hard work all the upkeep of being a woman - that is an absurd performance in itself," says O'Brien. But their performance art is not entirely serious: they seem more interested in provoking a reaction than making a defined statement. They often video their audience, asking them questions. And they are not singly focused on feminism – some of their protests are against racism and homophobia (O'Brien once dressed up as a terrorist at a music festival and asked people how it made them feel). They also want people to question why being Caucasian and blonde is as prevalent an aesthetic as being tall and slim. At one event they used the slogan: "Thin. Hairless. Nipped and Tucked. Plastic Fantastic. Is this all right? Or is it all white?"

On their website they say they want to show that "being a feminist doesn't mean you don't like men (we do) or don't shave (well, actually we don't shave sometimes) or don't have a sense of humour (we think we are very funny). It can be fun to be political and liberating to take risks." King says she would love to see "a mass movement with a sense of political conviction" which addresses some of the issues that they're worried about, in particular the way women are sexualised in popular culture.

Not all the attention the pair gets is positive. When I talk to several other people watching their performance, few understand what they are protesting about. And because they are two attractive young women wearing flesh-coloured body suits, a lot of the attention they get is good old-fashioned ogling – from men. Not that this seems to put them off. They plan more outdoor performances, and are also working on a stand-up routine, which features a sketch of two women who have had extensive plastic surgery. Most of all, they want to enjoy themselves. "I feel really liberated by flashing my merkin," laughs King.