They were protesting an amendment to the Communications Act requiring video-on-demand online pornography to follow tougher rules, already laid out by the British Board of Film Censors (BBFC) for DVDs sold in shops.
"While the measures won't stop people from watching whatever genre of porn they desire, as video shot abroad can still be viewed, they do impose severe restrictions on content created in the UK, and appear to make no distinction between consensual and non-consensual practices between adults," said the Facebook page of the protest organisers.
"This is a gateway to other laws being snuck in," she said.
A spokesman for the government's Department for Culture, Media and Sport said the legislation provided the same level of protection to the online world that already exists for the high street sales of DVDs.