By Agence France-Presse
Dissidents, rights lawyers and bloggers are routinely jailed in the communist country but rights groups and activists accuse the new government in place since last year of waging a particularly harsh crackdown on its critics.
Anti-China activist Tran Thi Nga, 40, was sentenced after a one-day trial in a heavily guarded courthouse in northern Ha Nam province, where dozens of supporters gathered outside, some holding signs calling for her release.
"The court convicted our client with an already prepared verdict," said lawyer Ha Huy Son, dismissing the charges as "groundless".
Nga, who was arrested in January, was best known for her anti-China activism, calling for Vietnam to stand up to its communist neighbour over a long-standing row in the disputed South China Sea.
She also rallied for land and labour rights and against police brutality. More recently she protested against the government's handling of a major toxic spill blamed on Taiwanese steel giant Formosa along Vietnam's central coast.
Nga was found guilty Tuesday for posting articles and videos online accusing "the communist state of violating human rights, and called for pluralism and the elimination of Article Four of the Constitution", which calls for a one-party state.
The charges carry a maximum sentence of 12 years.
AFP's request to attend the trial was not answered by local authorities, and several activists said they were harassed by plainclothes police outside the courthouse.
Nga's sentence follows the trial of prominent activist Nguyen Ngoc Nhu Quynh last month, better known by her pen name "Mother Mushroom", who was jailed for 10 years for anti-state propaganda.
Before Nga's verdict, New York-based Human Rights Watch lashed out at the government for its treatment of dissidents.
"The Vietnamese government consistently goes to extremes to silence its critics, targeting activists like Tran Thi Nga with bogus charges that carry a long prison sentence," said Phil Robertson, HRW's deputy Asia director.
Dozens of activists are imprisoned in Vietnam, where all independent media is banned and public protests are routinely broken up, often violently.
On Monday, activist Le Dinh Luong, 51, was arrested in central Nghe An province, accused of trying to overthrow the communist state, a crime punishable by death.