Last week, Kathy Sierra, a well-known software programmer and Java expert, announced that she had cancelled her speaking engagements and was "afraid to leave my yard" after being threatened with suffocation, rape and hanging. The threats didn't come from a stalker or a jilted lover and they weren't responses to a controversial book or speech. Sierra's harassers were largely anonymous, and all the threats had been made online.

Sierra had been receiving increasingly abusive comments on her website, Creating Passionate Users, over the previous year, but had not expected them to turn so violent - her attackers not only verbally assaulting her ("fuck off you boring slut . . . I hope someone slits your throat") but also posting photomontages of her on other sites: one with a noose next to her head and another depicting her screaming with a thong covering her face. Since she wrote about the abuse on her website, the harassment has increased. "People are posting all my private data online everywhere - social-security number, and home address - a retaliation for speaking out."

While no one could deny that men experience abuse online, the sheer vitriol directed at women has become impossible to ignore. Extreme instances of stalking, death threats and hate speech are now prevalent, as well as all the everyday harassment that women have traditionally faced in the outside world - cat-calls, for instance, or being "rated" on our looks. It's all very far from the utopian ideals that greeted the dawn of the web - the idea of it as a new, egalitarian public space, where men and women from all races, and of all sexualities, could mix without prejudice.

On some online forums anonymity combined with misogyny can make for an almost gang-rape like mentality. One recent blog thread, attacking two women bloggers, contained comments like, "I would fuck them both in the ass,"; "Without us you would be raped, beaten and killed for nothing,"; and "Don't worry, you or your friends are too ugly to be put on the black market."

Jill Filipovic, a 23-year-old law student who also writes on the popular blog, Feministe, recently had some photographs of her uploaded and subjected to abusive comments on an online forum for students in New York. "The people who were posting comments about me were speculating as to how many abortions I've had, and they talked about 'hate-fucking' me," says Filipovic. "I don't think a man would get that; the harassment of women is far more sexualised - men may be told that they're idiots, but they aren't called 'whores'."

Most disturbing is how accepted this is. When women are harassed on the street, it is considered inappropriate. Online, though, sexual harassment is not only tolerated - it's often lauded. Blog threads or forums where women are attacked attract hundreds of comments, and their traffic rates rocket.

Is this what people are really like? Sexist and violent? Misogynist and racist? Alice Marwick, a postgraduate student in New York studying culture and communication, says: "There's the disturbing possibility that people are creating online environments purely to express the type of racist, homophobic, or sexist speech that is no longer acceptable in public society, at work, or even at home."

Last year I had my own run-in with online sexism when I was invited to a lunch meeting with Bill Clinton, along with a handful of other bloggers. After the meeting, a group photo of the attendees with Clinton was posted on several websites, and it wasn't long before comments about my appearance ("Who's the intern?; "I do like Gray Shirt's three-quarter pose.") started popping up.

One website, run by law professor and occasional New York Times columnist Ann Althouse, devoted an entire article to how I was "posing" so as to "make [my] breasts as obvious as possible". The post, titled "Let's take a closer look at those breasts," ended up with over 500 comments. Most were about my body, my perceived whorishness, and how I couldn't possibly be a good feminist because I had the gall to show up to a meeting with my breasts in tow. One commenter even created a limerick about me giving oral sex. Althouse herself said that I should have "worn a beret . . . a blue dress would have been good too". All this on the basis of a photograph of me in a crew-neck sweater from Gap.

I won't even get into the hundreds of other blogs and websites that linked to the "controversy." It was, without doubt, the most humiliating experience of my life - all because I dared be photographed with a political figure.

But a picture does seem to be considered enough reason to go on a harassment rampage. Some argue that the increased visibility afforded people by the internet - who doesn't have a blog, MySpace page, or Flickr account these days? - means that harassment should be expected, even acceptable. When feminist and liberal bloggers slammed Althouse for her attack on me, she argued that having been in a photo where I was "posing" made me fair game. When Filipovic complained about her harassment, the site responded: "For a woman who has made 4,000 pictures of herself publicly available on Flickr, and who is a self-proclaimed feminist author of a widely-disseminated blog, she has gotten pretty shy about overexposure."

Ah, the "she was asking for it" defence."I think there's a tendency to put the blame on the victims of stalking, harassment or even sexual violence when the victim is a woman - and especially when she's a woman who has made herself public," says Filipovic. "Public space has traditionally been reserved for men, and women are supposed to be quiet."

Sierra thinks that online threats, even if they are coming from a small group of people, have tremendous potential to scare women from fully participating online. "How many rape/fantasy threats does it take to make women want to lay low? Not many," she says.

But even women who don't put their pictures or real names online are subject to virtual harassment. A recent study showed that when the gender of an online username appears female, they are 25 times more likely to experience harassment. The study, conducted by the University of Maryland, found that female user-names averaged 163 threatening and/or sexually explicit messages a day.

"The promise of the early internet," says Marwick, "was that it would liberate us from our bodies, and all the oppressions associated with prejudice. We'd communicate soul-to-soul, and get to know each other as people, rather than judging each other based on gender or race." In reality, what ended up happening was that, online, the default identity became male and white - unless told otherwise, you would assume you were talking to a white man. "So people who brought up their ethnicity, or people who complained about sexism in online communications, were seen as 'playing the race/gender card' or trying to stir up trouble," says Marwick.

And while online harassment doesn't necessarily create the same immediate safety concerns as street harassment, the consequences are arguably more severe. If someone calls you a "slut" on the street, it stings - but you can move on. If someone calls you a "slut" online, there's a public record as long as the site exists.

Let me tell you, it's not easy to build a career as a feminist writer when you have people coming up to you in pubs asking if you're the "Clinton boob girl" or if one of the first items that comes up in a Google search of your name is "boobgate". And for young women applying for jobs, the reality is terrifying. Imagine a potential employer searching for information and coming across a thread about what a "whore" you are.

Thankfully, women are fighting back. Sparked by the violent harassment of Sierra, one blogger started a "stop cyberbullying" campaign. This was picked up by hundreds of other bloggers and an international women's technology organisation, Take Back the Tech, a global network of women who encourage people to "take back online spaces" by writing, video blogging, or podcasting about online harassment.

It won't mean the end of misogyny on the web, but it is a start. Such campaigns show that women are ready to demand freedom from harassment and fear in our new public spaces. In the same way that we should be able to walk down the street without fear of being raped, women shouldn't have to stay quiet online - or pretend to be men - to be free of threats and harassment. It is time to take back the sites.