Bloggers and students are among 14 defendants, including two women, who heard their sentences read out in a packed courtroom in Vinh in Nghe An province, northern Vietnam.
At the end of the two-day group trial, 13 were sentenced to prison terms ranging from three to 13 years to be followed by house arrest of up to five years.
One of the defendants was given a three-year suspended sentence, making him vulnerable to re-arrest, Human Rights Watch said in a statement.
The defendants were arrested between August and December 2011 and held until trial this week.
"The conviction of yet more peaceful activists is another example of a government that is increasingly afraid of the opinions of its own people," Brad Adams, Asia director at Human Rights Watch, said.
"Instead of imprisoning critics, the Vietnamese government should be honoring them for their efforts to address the myriad problems facing the country that the government itself has also identified."
Some of the defendants participated in volunteer activities in their local neighborhoods in Vinh, including encouraging women not to have abortions, supporting the poor and people with disabilities, founding the Vinh Human Development Foundation and working to protect the environment.
The sentences are among the harshest given to any political dissident in Vietnam in recent years, a report by the BBC said.
Hundreds of policemen were among the public in the courtroom, which was packed because of the unusually large number of defendants on trial at once and because most were Catholics, the BBC said.
The U.S. Embassy in Hanoi issued a statement saying it is "deeply troubled" by the sentences because the defendants were exercising of their right to freedom of expression.
The sentences are "part of a disturbing human rights trend in Vietnam," the statement said. "We call on the government to release these individuals and all other prisoners of conscience immediately."
The defendants were accused of links with Viet Tan, a non-violent Vietnamese pro-democracy organization in the United States and with worldwide membership, including in Vietnam, but which Vietnam's government considers a terrorist group.
The 14 were charged after attending a Viet Tan training course in Bangkok and 11 were charged with being members of Viet Tan. Three were charged with actively participating in the organization, Human Rights Watch said.
Their convictions relied on loosely worded national security laws -- in this instance article 79 of the penal code, which vaguely prohibits activities aimed at overthrowing the government.
"It isn't clear if those convicted were targeted for alleged affiliations with Viet Tan, being members of the Redemptorist Church or simply for their activism," said Adams.
"Whatever the reason, the government appears despotic to its own people and the world when it says that someone who tries to uphold the rights of others is a threat to the state."
Human Rights Watch, as well as the U.S. Embassy, also called for the release of Le Quoc Quan who was arrested Dec. 27, nine days after he published an article entitled "Constitution or a contract for electricity and water service?"
He criticized Article 4 of the Constitution that states the Communist Party has the leading role in Vietnamese politics.
Vietnam has responded in the past to criticism over sentences for activists, in particular after a Ho Chi Minh City court jailed three bloggers in September for carrying out "propaganda against the Socialist Republic of Vietnam.
Nguyen Van Hai, known as Dieu Cay, 60, was sentenced to 12 years in prison on a charge of spreading propaganda against the state. Ta Phong Tan, 44, received a 10-year sentence and Phan Thanh Hai, 43 and a lawyer, was ordered to three years in jail.
All were members of the Free Journalists Club, an independent bloggers' website, and thought to have posted more than 400 articles, including 26 items containing anti-state comments.
The three defendants "received an open and fair trial," Foreign Ministry spokesman Luong Thanh Nghi said after international condemnation of the sentences.
"In Vietnam, every citizen's right to freedom is clearly stipulated in the Constitution and other legal documents," Luong said at the time.
"This right has always been respected. Just as in other countries, all law violations are punished in accordance with regulations of Vietnam's law and international human rights law, including the International Covenant on Civil and Political Rights."
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