On one side is the Church of Scientology, freshly boosted by a $10m donation from the actor Nancy Cartwright, the voice of Bart Simpson.

On the other is what the self-styled church has dismissed as a "pathetic" collection of "computer geeks": a maverick band of hackers who have launched an online war against the organisation.

The hackers were infuriated by the removal from YouTube last month of a Scientologists' video clip featuring Tom Cruise. They have alleged that Scientologists forced YouTube to remove the embarrassing footage, in which the actor hailed the religion as "a blast".

Cruise claimed: "We are the authorities on getting people off drugs, we are the authorities on the mind, we are the authorities on improving conditions ... we can rehabilitate criminals ... we can bring peace and unite communities."

Yet the church has appeared powerless to stop the online sabotage. Guerrilla action has so far included the temporary disabling of its international website and "Google bombing", a manipulation of the search engine which has resulted in the website being the first result returned by Google when users type "dangerous cult". Scientology's UK website has been unavailable and in the US the FBI were investigating what they said was the hoax dispatch of white powder in envelopes to 19 churches in the Los Angeles area.

Meanwhile, the intensity of the battle shows no signs of easing. A day of free speech protests have been planned outside Scientology centres around the world next Sunday, with campaigners mobilising on Facebook and YouTube.

Protesters are vowing to picket buildings in London, Birmingham, Manchester, Edinburgh and York on the British leg of the day of direct action, three weeks after a group of internet activists called Anonymous vowed to destroy the movement.

They said they were opposed to Scientologists' "speech-suppression tactics", which they claim include "frivolous" lawsuits and the use of copyright and trademark laws to silence free speech.

An Anonymous video posted on YouTube about the anti-Scientology campaign - called "Project Chanology" - has been watched more than 1.7m times. Protest sites against Scientology have also proliferated. Two Facebook groups have more than 3,500 members. A British organiser claims more than 1,000 people will join the UK demonstrations on February 10, with protests also planned in dozens of American cities and around the world, including Toronto, Vancouver, Oslo, Sydney, Melbourne and Dublin.

"I don't want them to get a foothold in the UK the same way as they have in other countries," said one demonstration organiser, a 21-year-old student who asked to remain anonymous in keeping with the protesters' aims. "They claim to be a church and a religion but they charge people to attend their sessions and they are a registered trademark - that doesn't strike anyone as a religion."

The organiser described the protesters as mainly a "youth movement" drawn from all walks of life. "At the start this was a hacker operation but it is more than that now. Scientologists say it's just a bunch of hacker geeks but that's going to be proved wrong on February 10."

In a series of warm-up rallies on Saturday, 100 people gathered outside a Scientology centre in Orlando carrying signs saying "Knowledge is Free". In the UK, half-a-dozen protesters leafleted shoppers and brandished a banner reading "Scientology Sucks" in Manchester.

Scientologists have fought back by hiring a specialist internet company to help defend its website. In the US, a Scientology spokesperson dismissed the campaigners as a "pathetic" group of "geeks".

But they played down the conflict yesterday. "We don't get into responding to such threats on the internet, particularly anonymous ones," said Janet Laveau, a spokeswoman for the Church of Scientology in Britain.

Laveau said the online publication by hackers of the Cruise video had caused a surge of interest in the movement. Presenting "these selective and out-of-context excerpts with the intent of creating both controversy and ridicule nevertheless resulted in people searching for and visiting Church of Scientology websites," she said. "Those wishing to find out the Church of Scientology's views and to gain context of the video have the right to search official Church websites." In a US statement, the organisation said it did not threaten websites for posting the Cruise video.

But Scientologists have previously mounted legal challenges against other internet publishers. A 10-year legal campaign against a Dutch writer, Karin Spaink, and a number of internet service providers ended in defeat for the Scientologists in court in the Netherlands in 2005. Spaink and others had posted documents from a disaffected former Scientologist that were claimed to be the secretive official teachings.

According to Andreas Heldal-Lund, a free-speech campaigner from Norway and critic of Scientology, the "war" has been won by internet activists - with the wide dispersal of negative publicity about Scientology. Heldal-Lund welcomed the demonstrations but condemned the sabotage of Scientology websites.

"One of the biggest arguments against Scientology is they are a threat to free speech and here they can say people are hitting back at their free speech. It ruins our argument."