Three human rights activists were detained after attending a meeting with European Union diplomats in Hanoi. Two were released after questioning, but one, the journalist, Pham Doan Trang, was held late into the night on Thursday.
Trang and fellow activist, Bui Thi Minh Hang, were detained by about ten police officers as they were walking together after the meeting.
They were taken to different locations. Ms Trang had her communication equipment confiscated and was unable to inform colleagues that she had been released, sparking fears for her safety.
The veteran campaigner and former entrepreneur, Dr Nguyen Quang A, was also briefly detained after the meeting.
European Union diplomats invited four activists to the meeting after announcing that an EU delegation will visit Vietnam early next month for their annual dialogue on human rights.
Dr Quang A wrote on his Facebook page that it was the 14th time this year that he had been arrested, an indication of the increasing harassment, threats and prosecutions being suffered by dissidents.
The Vietnamese authorities regularly try to prevent foreign delegations from meeting government critics who want to draw attention to growing repression in the country.
Trang is a journalist and blogger known for her fearless denunciations of human rights abuses. She is regularly followed by police agents and subjected to threats and harassment for her work.
Vietnam’s small community of civil society activists is concerned that President Trump’s lack of interest in their plight, amply displayed during his recent visit, is emboldening the authorities in their crackdown.