Amnesty International is calling on internet users to join its campaign to reclaim the web as a place of freedom.
The lobby group names Cisco, Google, Microsoft, Nortel Networks, Sun and Yahoo! as companies who have co-operated to censor the internet or to help track down individuals.
Amnesty wants users of these services to complain directly - by email or on support forums - about these companies support of repression in China and elsewhere. The group also wants the companies to "come clean" on their policies - they want the companies named to make public all agreements with the Chinese government and publish a list of words excluded from web and blog searches. Amnesty also wants the companies to publicly call for the release of "cyber-dissidents" currently imprisoned.
Amnesty accuses the firms of hypocrisy.
Kate Allen, Amnesty's UK director, said: "There is a huge gulf between the companies stated values and their actions. Yahoo!, Microsoft and Google need to stop denying their culpability, acknowledge where their responsibilities lie, and begin to focus on solutions in China."
Yahoo! handed over confidential information which led to the imprisonment of two journalists - Shi Tao and Li Zhi - who have both been adopted as Amnesty Prisoners of Conscience.
Microsoft shut down a blog run by a New York Times researcher Zhao Jing following a request from the Chinese government.
Google is offering a censored version of its search engine for China.
Microsoft gave Israeli authorities the Hotmail account of Mordechai Vanunu - the man who blew the whistle on Israel's nuclear weapon programme. He is out of prison but banned from contacting the foreign media.