Freedom of expression is severely restricted in communist Vietnam, where independent media is banned and dissidents are routinely thrown in jail.
The government also has a long history of harassing bloggers and activists, with awareness of violent attacks growing in recent years as dissidents turn to social media to share accounts of bloody wounds and bruised limbs.
In a new report Monday, Human Rights Watch (HRW) described 36 incidents of physical assault on dissidents by men in civilian clothing across Vietnam between January 2015 and April 2017.
"While the precise links between the thugs and the government are usually impossible to pin down, in a tightly controlled police state there is little or no doubt that they are aligned with and serving at the behest of state security services," the report said.
In many cases, the violence took place publicly in the presence of uniformed police officers who did not intervene, it added.
"The fact that thugs abducted activists in broad daylight, forced them into vans, and beat them demonstrates the impunity with which activists are persecuted," said HRW Asia director Brad Adams.
The US State Department and other rights groups such as Amnesty International have previously reported harassment and abuse by plainclothes police.
The Vietnam government did not respond to a request for comment on the report.
Dissident La Viet Dung told AFP he was attacked by six men in July 2016 after playing a football match with other activists in Hanoi.
He was smashed in the face with a brick, and still bears a scar above his eye today.
"The attacks on me and others came from plainclothes men," said the 42-year-old software developer, who continues to face surveillance by authorities.
Fellow activist Pham Doan Trang, 38, said she has faced harassment at the hands of plainclothes police several times, including being physically assaulted at a protest in April 2015.
Activists also reported being punched, beaten with metal tubes or dragged into vans and abandoned in deserted areas, HRW noted.
Because the attackers are unidentified, victims say they have no legal recourse if they are harassed.
The rights watchdog called on international donors and trade partners to demand an end to the abuse in Vietnam, a leading recipient of foreign aid.
One of the fastest growing economies in the region, it has also attracted heaps of private foreign investment in recent years.
But moves to liberalise its economy have not been accompanied by a loosening of civil liberties.
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