The human rights campaigner and lawyer, Nguyen Van Dai, and three of his associates, have been attacked and robbed by masked men during a visit to fellow activists in central Vietnam.
They were in a taxi heading back to Hanoi when they were intercepted by men they identified as plainclothes police officers driving two unregistered cars and five motorcycles.
They said as many as twenty officers rushed at them, dragged them from the taxi and began beating them with metal bars.
Two of the group managed to run off and call for help, but Dai was cornered and subjected to a violent beating. He was partially stripped, robbed of his wallet, phone and shoes, and left beside a beach.
The other activists, Vu Van Minh, Ly Quang Son and Le Manh Thang, also suffered cuts and bruises in the assault.
Dai is a former political prisoner and vice-president of the Brotherhood for Democracy.
The encounter with the police had begun earlier when officers had attempted to disrupt a meeting the Hanoi activists were having with local civil rights groups and Catholic campaigners in Nghe An province.
The previous day, Saturday, Mr Dai had been been prevented from attending another meeting to mark International Human Rights Day at a branch of the Redemptorist Church.
The assault is the latest in a series of violent attacks on civil rights campaigners.
Plainclothes men, or civilian toughs hired by the police, have been implicated in a number of the assaults, including recent violent attacks on lawyers and labour rights activists.
Mr Dai was released earlier this year after four years of house arrest, which followed four years imprisonment for criticising the government.
He said that police had largely stopped harassing him after the intervention of foreign diplomats in March, and he was allowed to travel more freely in the months that followed.
The latest assault will be seen as another sign of a more repressive approach by the government.
Some activists have pointed to the role of the public security minister, Tan Dai Quang, who is tipped for a senior leadership post during the Communist Party congress expected next month.
Quang recently identified some 60 civil society organisations, condemning them as counter-revolutionary, and outlining the tactics of the police to contain activists and restrict their activists.