Prominent activists ranging from Edward Snowden to Pussy Riot have planned to spread online protests against growing government restrictions on citizens’ expression, marking an annual world day against cyber censorship. In one message, Snowden, will tell the public: “Even if you’re not doing anything wrong, you’re being watched and recorded.” The Chinese artist Ai Weiwei will also take part. One of his messages will read: “Without freedom of speech there is no modern world, just a barbaric one.”

The online messages are part of activities to mark World Day Against Cyber Censorship, an annual online event started in 2008 by Reporters Without Borders and Amnesty International. This year, Amnesty has teamed up with the controversial online advertisement-blocking company AdBlock to bring activists’ voices to the public.

In 2015, Amnesty recorded people being arrested in more than 16 countries for saying or doing online what would in most countries be regarded as normal freedom of expression. Gabriel Cubbage, chief executive of AdBlock, said the company would show Amnesty’s online banners in place of ads to its 40 million users for one day only.

The rise of advert blocking software has prompted great anxiety in the online news media, which relies on ad revenue, especially when content is free.