The Hanoi police released six democracy campaigners in the middle of the night after holding some for more than fourteen hours.
The move followed violent scuffles outside police headquarters in Hai Ba Trung District after some thirty protesters went there to demand the release of their fellow activists.
The protesters said there was panic as they were attacked by officers and plainclothes agents. They said at least two were hurt in the melee. The blogger, Doan Trang, said she was hit in the face and left with a bleeding mouth.
Witnesses said a recent medical graduate, Dinh Thi Phong Thao, suffered torn clothes and bruised arms and was dragged onto the wet pavement.
The police had earlier denied they made any formal arrests of the democracy activists.
But word quickly spread through social media that six people, some associated with a new online video news service, had been picked up at their homes early on Wednesday morning.
Amongst them was Le Thi Yen, the 23-year-old anchor of the channel which is broadcast on You tube under the name Luong Tam (Conscience).
The site specialises in stories about human rights and democracy and aims its news at a broad cross section of Vietnamese society.
The former political prisoner and founder of the Brotherhood for Democracy, Nguyen Van Dai, said some of those held had all their computers, mobile phones and other electronic equipment confiscated.
Most of those detained had taken part in civil society campaigns, including the successful challenge to the Hanoi City government in the spring, over its plan to cut down thousands of the city’s famous old trees.
Luong Tam broadcast its first edition on August 19 and produces a bulletin of news stories that are not covered by the state controlled mainstream media.