They've become the sworn enemies of a controversial religion, famous for unsettling online video attacks and protesting en masse in creepy masks.
But the elusive internet group known as Anonymous didn't start out with a concrete plan to bring down Scientology. Before they got serious, Anonymous members say, they were only in it for laughs.
In the years before "declaring war", the group's stock-in-trade was the creation of internet jokes revolving around, for instance, kitsch 1980s singer Rick Astley.
Anonymous's transformation from pranksters with a tendency to overstep the mark to a group serious enough to scare Scientology is an intriguing story of the virtual world's increasing influence on the "real" world.
Its critics accuse it of being a malevolent force. But Anonymous is not a group in the traditional sense it has no leaders, no head office and no agreed-upon agenda.
It resides in anarchic, occasionally disturbing online communities that are also engaged and knowledgeable.
Anonymous can be cruel. Sometimes it's a hotbed of creativity; at other times it's simply inane. And it may be responsible for creating your favourite online in-joke.
When a video of Tom Cruise spruiking Scientology's benefits was posted to YouTube in January this year, it quickly became an internet hit.
The video was intended for internal use, with Cruise manically explaining how Scientologists had "the ability to create new and better realities" for humanity. Scientologists alleged the video's posting was a copyright violation and issued YouTube with a take-down notice. The website duly complied.
But members of internet communities such as"4chan" and "eBaum's world" saw it as an attack on internet freedom and took exception.
As one Anonymous member (we named him Guy Fawkes, after the mask he insisted on wearing) told ninemsn: "[They] brought it to our attention, which was a big mistake."
Anonymous launched a raid on Scientology's website on January 21, overrunning the church's servers and shutting them down.
On the same day, it released a now-infamous video threatening to systematically destroy Scientology. Featuring a distorted voice and sped-up stock footage, the video was disturbing enough to make people take notice. "Project Chanology" was born.
"Anonymous has … decided your organisation should be destroyed, for the good of your followers, for the good of mankind and for our own enjoyment,” the voice said.
"At first it was just this big joke, yeah, lulz, we're going to protest against Scientology," Guy Fawkes told ninemsn.
"Then people who actually had really good reasons to protest Scientology got involved and started educating us, at the same time as we were educating ourselves, and we discovered we had a pretty good case on our hands."
By the time the group took to the streets on February 10, mainstream media outlets were all over the story. But while Anonymous has been widely discussed, there is still little understanding of its actual nature.
To get to the heart of Anonymous, you need to understand the culture that spawned it.
'WE ARE LEGION'
A visit to 4chan.org's /b/ forum is a baffling experience for the uninitiated. You could see an image of a dog wearing false teeth or a horribly mutilated dead body. You'll definitely see hardcore pornography.
/b/ is an image board, where members (who call themselves b/tards) post pictures of any kind. Its only taboo is child pornography. Other users respond to these pictures - popular threads stay at the top.
If the origins of Anonymous can be traced to any one point, it's this almost lawless online space, and other forums like it.
According to our Sydney-based Anon and b/tard Guy Fawkes, /b/'s raison d'etre is "memes".
The concept of memes originates from the philosopher Richard Dawkins, and refers to units of information (for instance, a catchy tune) that propagate through a culture.
On the internet, the term has become shorthand for the jokes, videos and catchphrases that spread virally online.
Some memes that originated or were popularised on 4chan are now famous: for instance, LOLcats, Tay Zonday and Rickrolling (click here for explanation).
Even the term "Anonymous" is really a meme a descriptive term for 4chan, 7chan and other communities, with its own mythology and aesthetic.
"Every time you've ever posted anything on the internet without stating your name, you've been Anonymous, and that's all that defines Anonymous," Guy says.
Anonymous "raids" targets that catch its attention. It may overload a website's servers with a co-ordinated attack, or hack into a hapless individual's MySpace page.
One such incident prompted Fox News's local channel in Los Angeles to run a shamelessly sensational story on the group, which it called an "internet hate machine".
The story alleged that Anonymous made credible threats to bomb football stadiums across the US. It even included footage of an exploding SUV to ram home the point.
While such portrayals are exaggerations, there's no doubt that Anonymous can be nasty. It takes on big targets like Scientology, but it also mocks much weaker individuals (such as "Chin-Chan", a girl with an unfortunately sunken chin).
Guy Fawkes compares the internet to the streets.
"I don't think Anonymous sets out to be cruel to people … If you act like a douche bag out in the street, you're going to get treated like a douche bag," he says.
"[But] any illegal act acted out by someone on the internet is denounced by Anonymous … We're not terrorists. We're not bullies."
Scientologists in Australia say they've been the victims of bomb threats, death threats, break-ins and vandalism since Anonymous began its campaign.
Vicki Dunstan, a spokeswoman for Scientology, said the initial attack in January left church members bewildered.
"There was no single person or organisation or anyone you could pin it to, it was just a generality. Then we became aware of the video on YouTube," Ms Dunstan says.
"The sort of hate speak that you see coming out of Anonymous, it's quite threatening, it's alarming and it incites others to take action against the church."
Ms Dunstan believes many Anonymous members are badly informed about Scientology.
"Some of their allegations are over four decades ago and the majority of these people have no first-hand experience of the church," she said.
Anonymous members reject these accusations, and say their behaviour at the three Sydney protests so far has been beyond reproach.
There's more than a whiff of paranoia in the air with both sides accusing their opponents of orchestrating smear campaigns and manufacturing evidence.
JUST ANOTHER MEME?
Scientologists are hopeful that, after almost four months, Anonymous is losing its stomach for a protracted battle.
"I think quite a lot of people who are in the group are just in it for the 'lulz', and they get recruited for it and it's a bit of a joke," Ms Dunstan says.
But Sydney Anon Guy Fawkes says he, for one, is in it for the long run.
"I think what is important about Anonymous is that it works, I don't know how it works but it works," he says.
"The light-hearted side of Anonymous is being left on the internet now, which I think is important. Because you can't deal with a serious issue with a light heart.
"I don't think we're going to get bored any time soon … it's going to go for a long time before it either fizzles out or ends abruptly."
Anonymous's next worldwide protest against Scientology, dubbed "Battletoad Earth: Operation Fairgame Stop", will take place on May 10.