Masked demonstrators gathered outside London's Church of Scientology in protest against the organisation.
The Church of Scientology said the group were cyber terrorists
The group, called Anonymous, said they wanted to highlight the organisation's "inherent flaws" and "fight for freedom of knowledge and information".
The City of London Police said about 200 people took part in the peaceful protest in Queen Victoria Street.
After about two hours the protestors moved to the Scientology Recruitment Centre in Tottenham Court Road.
Similar gatherings took place outside Scientology Centres across the UK and in countries including Australia, Canada and the US.
In a video statement broadcast online, the organisers said: "The idea of Anonymous is to systematically dismantle the Church of Scientology in its present form."
They said they were campaigning against "toxic ideals" and a "corrupt leadership", not the beliefs of members.
Anonymous posted a Scientology video of Tom Cruise on the internet last month, which was taken down after complaints about copyright infringement.
Janet Laveau, from the Church of Scientology, said the protest group were "cyber-terrorists" motivated by "religious bigotry".
She said: "The actions of Anonymous will not interrupt the church's normal activities serving its parishioners and the community, and the church is working in co-ordination with local authorities to minimize the negative impact of this mask-wearing, cyber-terrorist group."
The protests were held on 10 February to mark the birthday of Lisa McPherson, an American scientologist who died in 1995 while under the care of the organisation.