Vietnamese authorities have been accused of creating a climate of fear among the country's internet users.
Human rights organisation Amnesty International says the Vietnamese government is using online informers to keep track of web users.
Several campaigners for democracy have been jailed for discussing their ideas on websites and in e-mails.
Amnesty made the claims in a report ahead of next week's United Nations meeting on governing the internet.
Activists detained
The internet is booming in Vietnam. Online cafes can be found in even the smallest towns.
It has given a big boost to the country's small dissident movement - allowing them to communicate with each other and with supporters abroad.
In response, the security forces have blocked access to certain websites and targeted dissidents using the net.
Over the past few months a number of activists have been detained and their computers searched and confiscated. One, Trong Quoc Huy, has not been heard of since he was arrested in mid-August by plain-clothes police while sitting in an internet cafe.
Another, Nguyen Vu Binh, a journalist, was detained four years ago for passing information through the internet to overseas Vietnamese groups.
"People in Vietnam can be thrown in jail for the click of a mouse"
Foreign governments have been pressing for him to be released in an unusual prisoner amnesty announced for the end of this month, shortly before several world leaders arrive in Vietnam for the annual Asia-Pacific summit.
Amnesty International are highlighting Vietnam's policies ahead of a meeting organised by the United Nations to discuss how to regulate the internet in the future.
"People in Vietnam can be thrown in jail for the click of a mouse," said Amnesty International UK's director Kate Allen.
"But a growing number of brave activists are defying internet repression and using the web to fight for human rights.
"And the global nature of the internet means that people all over the world can help call for greater online freedoms in Vietnam - and support our campaign to free Vietnamese cyber-dissidents."
The example of Vietnam shows how different governments take very different attitudes to questions of free speech and censorship online - an issue which threatens to fracture the current international consensus on regulating the internet.
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