HANOI, Dec 16 (Reuters) - A Vietnamese lawyer badly beaten 10 days ago by unknown attackers was arrested by police on Wednesday for anti-state "propaganda", the latest incident in what rights groups are calling an alarming crackdown on government critics.

Despite sweeping reforms to its economy and increasing openness towards social change, including gay, lesbian and transgender rights, Vietnam's ruling Communist Party retains tight media censorship and zero tolerance for criticism.

The authorities have been accused of using vague laws to stifle bloggers and activists who are getting more exposure from the proliferation of the Internet and social media in Vietnam, which has one of Asia's highest concentrations of Web users.

Human rights activist Nguyen Van Dai, who previously served jail time for the same offence, will be investigated "according to law", said the website of the Ministry of Public Security, the powerful police-run entity charged with curbing perceived threats to the one-party state.

Dai, the 47-year-old founder of the Committee for Human Rights in Vietnam, and three associates, were brutally beaten by about 20 unknown men wielding sticks after they participated in a human rights workshop, activists say.

A representative of the United Nations human rights commissioner condemned that attack and said activists' allegations that it was carried out by plain-clothes police should be fully addressed.

Vietnam's treatment of its detractors has put Western countries in an awkward position, especially the United States and the European Union, as they engage closely with the country through trade pacts, such as the Trans-Pacific Partnership.

The EU was criticised for refusing to carry out a human rights impact assessment before it concluded a free trade agreement with Vietnam a few months ago.

The New York-based Human Rights Watch says there has been a reduction this year in what it calls politically motivated trials and convictions, but called that an attempt to gain favour while trade deals were being finalised.

Vietnam is now holding at least 130 political prisoners, the group says. (Reporting by Martin Petty; Editing by Clarence Fernandez)

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