The MasterCard website was forced offline for several hours today, following an online assault led by a shadowy group of hackers protesting against the card issuer's decision to block payments made to the WikiLeaks website.

The "distributed denial of service" attack was apparently orchestrated by a "hacktivist" group calling itself Anonymous, which has in recent days temporarily paralysed the websites of Post Finance, the Swiss bank which closed WikiLeaks frontman Julian Assange's account, and the website of the Swedish prosecution office.

Twitter is next in its sights, following allegations that the social networking site is "censoring" visibility of the breadth of discussion of WikiLeaks by preventing it from appearing in Twitter's "trends". Twitter has denied that it is doing this, saying its systems identify topics that are "being talked about more right now than they were previously" – which doesn't include WikiLeaks.

But who, or what, is – or are – Anonymous?

A 22-year-old spokesman, who wished to be known only as "Coldblood", told the Guardian that the group – which is about a thousand strong – is "quite a loose band of people who share the same kind of ideals" and wish to be a force for "chaotic good".

There is no real command structure in the group, the London-based spokesman said, while most of its members are teenagers who are "trying to make an impact on what happens with the limited knowledge they have". But others are parents, IT professionals and people who happen to have time – and resources – on their hands.

The group has gained notoriety for its attacks on copyright-enforcement agencies and organisations such as the Church of Scientology.

Anonymous was born out of the influential internet messageboard 4chan, a forum popular with hackers and gamers, in 2003. The group's name is a tribute to 4chan's early days, when any posting to its forums where no name was given was ascribed to "Anonymous". But the ephemeral group, which picks up causes "whenever it feels like it", has now "gone beyond 4Chan into something bigger", its spokesman said.

The membership of Anonymous is impossible to pin down; it has been described as being like a flock of birds – the only way you can identify members is by what they're doing together. Essentially, once enough people on the 4chan message boards decide that an issue is worth pursuing in large enough numbers, it becomes an "Anonymous" cause.

The group counts the current campaign in support of WikiLeaks as "probably one of [its] most high profile yet". The group gained notoriety more recently for a number of sustained assaults against the sites of US music industry body RIAA, Kiss musician Gene Simmons, and solicitors' firms involved in lawsuits against people suspected of illegal filesharing. In early 2008, Anonymous launched a campaign against the Church of Scientology, bringing down related websites and promising to "expel" the religion from the internet.

"We're against corporations and government interfering on the internet," Coldblood added. "We believe it should be open and free for everyone. Governments shouldn't try to censor because they don't agree with it.

"Anonymous is supporting WikiLeaks not because we agree or disagree with the data that is being sent out, but we disagree with any from of censorship on the internet. If we let WikiLeaks fall without a fight then governments will think they can just take down any sites they wish or disagree with."

The spokesman said Anonymous plans to "move away" from DDoS attacks and instead focus on "methods to support" WikiLeaks, such as mirroring the site. "There's no doubt in [Anonymous members'] mind that they are breaking [the] law," he said of the latest attacks. "But they feel that there's safety in numbers."

Anonymous refused to say whether it would target government-owned websites next, but warned: "anything goes."