Amnesty International is urging UK users of Yahoo, Microsoft and Google to e-mail the companies asking them to change the way they operate in China.
Amnesty says key words are being filtered by web giants in China
The human rights organisation says the companies are aiding internet censorship in the Communist state.
And it is asking them to reveal which words they have banned from blogs or web searches in China.
The internet companies say they are helping the people of China by making information more freely available.
But Amnesty says they are helping to reinforce censorship by the Chinese government.
Amnesty International UK director Kate Allen said: "Yahoo, Microsoft and Google claim they are obeying local laws when in fact they are succumbing to political pressure.
"We want them to hear from customers here in the UK.
"If enough people tell them they are not happy with their actions in China, we hope it will make them think again."
But the technology companies argue they are simply working within existing laws.
A Google spokesman said: "People in China have access to far more information today than they did five years ago - and by being in China we will help further that development."
Yahoo spokeswoman Mary Osako said: "We also believe our presence significantly benefits a country's citizens through access to services and information."
A Microsoft spokesman said: "We believe in freedom for users to connect to the people and information that is important to them, but Microsoft will continue to comply with local laws of the markets in which we do business.
"While this is a complex and difficult issue, we remain convinced it is better for Microsoft and other multinational companies to be in these markets with our services and communications tools, as opposed to not being there."