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The latest high-profile case involves Bui Thi Minh Hang, a 47-year-old activist, who was sentenced to two years in a “re-education camp” near Hanoi in November after participating in a number of rare public protests against perceived Chinese aggression in contested areas of the South China Sea.

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Her arbitrary detention, news of which only emerged recently, has brought renewed focus on Vietnam’s use of highly secretive re-education camps, which many observers presumed had been closed when the south-east Asian nation normalised relations with the US.

The US embassy in Hanoi on Thursday called on the Vietnamese government to release Ms Hang and all other political prisoners, saying their detention violated the international human rights conventions of which Vietnam is a signatory.

“No person should be imprisoned for exercising their freedoms of expression or peaceful assembly,” the embassy said.

Under Vietnamese law, the police have wide-ranging powers of arbitrary detention for behaviour that is deemed to be damaging to society but not serious enough to justify criminal prosecution.

Vietnam’s Communist-led government regularly detains democracy campaigners, legal activists and peaceful protesters. But foreign diplomats say this has been stepped up over the last year because of the growing influence of internal security officials within Vietnam’s closed political system. Top leaders fear that the turbulent economic climate has increased the risk of social disorder.

“The police and local governments can send people to these camps on an arbitrary basis with no legal process or effective right of appeal,” said one Vietnamese lawyer, who did not want to be named out of fear of political repercussions. “This is a total violation of Vietnam’s international human rights commitments.”

The authorities became increasingly nervous as events of the Arab Spring unfolded and following widespread protests in Communist neighbour China, foreign diplomats say.

The Ministry of Public Security, which has two members on Vietnam’s 14-man Politburo, the country’s top decision-making body, is “very scared,” about threats to stability and wants to send a warning signal to other dissidents, said one Western diplomat.

A group of prominent intellectuals sent an open letter to Truong Tan Sang, Vietnam’s president, last month calling for Ms Hang to be set free.

Phil Robertson, the deputy Asia director of Human Rights Watch, a US-based campaign group, welcomed the US statement but called on Western governments to increase the diplomatic pressure on Vietnam.

There are no publicly available statistics on how many re-education camps Vietnam operates or how many people are detained in them.

Those detained in the camps, called “pupils” by the state-controlled media, are subject to training in Communist ideology and forced labour.

Vietnam has also come under increasing international criticism for arbitrarily detaining tens of thousands of female sex workers, drug addicts and troubled children in similar camps, where they are also forced to work, processing cashew nuts or sewing mosquito nets.

By Ben Bland - The Financial Times - January 5, 2012