This weekend's anti-war demonstration was almost certainly the largest coordinated political protest the world has seen. Events began in Melbourne, Australia, and then erupted in hundreds of cities across the world like a global Mexican wave before ending in San Francisco 48 hours later.

While estimates range from six to 12 million people taking part, February 15 - or F15 in activist parlance - underlines the extent to which the dynamics of protest politics have been transformed by the internet.

As a press officer for the Stop the War Coalition in the run-up to Saturday's march, I experienced first-hand how the internet has allowed tiny political groups with virtually no resources to mobilise millions of people.

The press campaign would not have existed without the internet. By using email for press releases, we could send immediate responses to government statements at no cost. We could direct journalists to our website. And the internet provides a vital resource for checking discrepancies in government statements. The revelation that a government dossier was cut and pasted from a 10-year-old thesis would never have made the headlines had the original author not read the dossier on the internet. The web exposes government information to unprecedented scrutiny.

The internet has dramatically effected the way Stop the War Coalition is organised.

"A major part of campaigns in the past was stuffing envelopes. That used to take literally days," says Andrew Burgin, one of the founders of Stop the War. "So campaigns in the past always required a much bigger labour force."

Using mailing lists and its website, the central office communicates with a rapidly growing network of local groups that provide much of the movement's organisation. Those local groups communicate with their members and the wider movement through their own mailing lists, group text messages and local websites. The groups also run their own press campaigns with local media.

My local group is in Dulwich, South London - hardly a hotbed of revolutionary fervour. But meetings draw people from every ethnic background, class, age and political persuasion, who you could hardly imagine meeting in any other circumstance.

The founder of the group, Mike Healey, has also researched the social effects of the internet as a lecturer for Westminster University.

"We've used the internet to build the group," he says. "There's a lot of talk of virtual communities and what's happened here is one example.

"But the community only exists to mobilise - not to just chat online."

This distributed structure has proved to be infinitely extensible, with more and more local groups continuing to be formed and the central office remaining minimal - it is run by less than 10 people.

But for the anti-war movement the internet is only a means to an end. The reason people get involved is not for online discussions, but for offline protest such as Saturday's march. The internet simply makes that process more accessible to people who would not normally get involved in politics.

Now we're about to launch a global anti-war website linking the national groups together so we can coordinate further protests in the coming months.

The web has allowed Stop the War to connect with people in a way politicians have failed to do. The much hyped age of online politics has finally arrived .