(Reuters) - Thirteen political activists were found guilty of anti-state crimes in Vietnam on Wednesday and sentenced to prison, a ruling condemned by rights activists who saw it as part of a crackdown on dissidents in the communist country.

Relatives of the defendants and several Catholic blogs said the 13, including bloggers and members of a Catholic church, were sentenced to terms ranging from three to 13 years. Another accused received a suspended sentence.

Court officials declined to provide details of the verdict, which was read out after a two-day hearing during which large numbers of police were deployed around the courthouse.

The court in Vinh, 300 km (190 miles) south of Hanoi, found them guilty of "carrying out activities aimed at overthrowing the people's administration", a charge under Article 79 of the penal code that can carry the death penalty.

"Article 79 is a very draconian charge," said Phil Robertson, deputy Asia director at Human Rights Watch. "There is nothing to indicate the defendants intended to overthrow the government."

"This trial is in the middle of a deepening crackdown that's been gradually picking up speed in the past year, year and a half. They're mowing down the ranks of activists in Vietnam," he said.

In a statement, the U.S. embassy in Hanoi said it was "deeply troubled" by reports of the convictions.

"The government's treatment of these individuals appears to be inconsistent with Vietnam's obligations under the International Covenant on Civil and Political Rights, as well as the provisions of the Universal Declaration of Human Rights relating to freedom of expression and due process," it said, calling for all prisoners of conscience to be freed.

The 14 defendants were arrested between August and December 2011 and held for more than a year before standing trial.

Rights groups say they are peaceful protesters and advocates of workers rights and democracy, plus supporters of other imprisoned activists.

Government officials were not available for comment.

FEAR OF PROTESTS

Eleven of the defendants were identified in an official indictment as members of Viet Tan, an outlawed pro-democracy group based in the United States. The activities deemed subversive included attending a digital security workshop in Thailand.

"People in Vietnam have the right to participate in the political affairs of the country. They have the basic right of belonging to any political organisation they choose," Duy Hoang, a spokesman for Viet Tan, told Reuters.

"No one is accused of doing anything that is actually a 'wrong' activity. They are being persecuted," he said.

Hoang declined to say whether any of the defendants were members of the party.

Dang Ngoc Minh and her daughter Nguyen Dang Minh Man were accused of painting the slogan "HS.TS.VN" on a school. According to the defendants, that meant "Hoang Sa, Truong Sa, Viet Nam" -- or "the Paracel and Spratly Islands belong to Vietnam".

Those islands are also claimed by China in a territorial dispute that flared up anew in 2012. The Vietnamese government agrees with the slogan, that the islands belong to Vietnam.

"Vietnamese authorities haven't been able to say why this is bad," Robertson said of the slogan.

"Part of the reason the government cracked down on protests related to policies on China is that it fears such protests will get out of control and morph into something else."

A crackdown could have international trade repercussions.

"There's opposition in the U.S. to extending economic benefits to a country engaged in activity so antithetical to its values," said Allen Weiner, a senior lecturer in international law at Stanford Law School.

"The government of Vietnam is conducting a legal process which is completely non-transparent. The courts are being used as an instrument of state repression rather than honestly adjudicating guilt or innocence," he said.

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