The Vietnamese government is cracking down on internet use and has jailed at least 10 people for online activity, according to the human rights organisation Amnesty International.
There are about two and a half million Vietnamese internet users
A report released by Amnesty accused Vietnam of using national security legislation to prosecute those who criticise the government on websites or in e-mails.
"In Vietnam, clicking on the 'send' button carries the risk of being sent to prison and having your friends and family put under 24 hour surveillance," the report said.
There are thought to be around two-and-a-half million internet users out of a population of 80 million in Vietnam.
Amnesty said the government was keen to promote internet use as part of the country's economic development, but was suspicious of its other uses.
It said that websites had been blocked, e-mails monitored and internet cafe owners encouraged to report the activities of users.
"The Vietnamese government appears unwilling to recognise that the internet can only be a tool for development and prosperity if the right to freedom of expression and information is respected fully in both law and practice," Amnesty said.
The organisation estimated that since 2001, at least 10 people using the internet to criticise government policies through emails or websites had been arrested.
Six of these so-called "cyber-dissidents" have received prison sentences.
Among others, the report highlighted the case of Nguyen Vu Viet and Nguyen Truc, who are brothers, and their sister Nguyen Thi Hoa.
They allegedly sent information to overseas Vietnamese groups about religious freedom in Vietnam.
All three of them were charged with "abusing democratic freedoms" and sentenced to between three and five years in prison, although an appeal hearing is scheduled for 28 November.
Amnesty called on the Vietnamese government to unconditionally release all those imprisoned for expressing their views, and the removal of restrictions on the internet.