The police broke up an attempted demonstration by supporters of pro-democracy activists, as a major political trial got underway in Hanoi.
Police and plainclothes agents blocked the march by a small group of protesters, who carried placards saying that democracy is not a crime. They were seen bundling a number of the marchers into waiting buses.
The high profile human rights lawyer, Nguyen Van Dai, and five others went on trial in the morning on charges of attempting to overthrow the government.
They could face anything from 12 years imprisonment to the death penalty under article 79 of the penal code, if they are convicted at the end of the two-day trial.
Mr Dai, aged 48, appeared older and greyer than before his arrest, as he took his place alongside his co-defendants in the court. He has been held for some 27 months in pre-trial detention in conditions that other defendants often describe as harsh and deliberately isolating.
He was arrested near his home in December 2105, along with his assistant, Le Thu Ha, after meeting European Union diplomats.
He had been badly beaten up and dumped by a roadside by pro-government thugs the week before.
Ha also appeared in the dock beside him, along with four other activists, Pham Van Troi, Nguyen Trung Ton, Truong Minh Duc and Nguyen Bac Truyen, who were arrested last year and accused of being members of the Brotherhood for Democracy pressure group.
The indictment said the defendants has taken advantage of the fight for democracy, human rights and civil society to conceal their true subversive purpose.
It said they had also sought financial help from overseas to further their anti-state activities.
Western governments and international human rights groups see Dai and his associates as prisoners of conscience, who have used peaceful methods to try to encourage political change in Vietnam.
Dai is seen as a courageous campaigner, who has refused to give up the struggle despite an earlier term of harsh imprisonment.
He appears to have alarmed the authorities with his outreach to religious communities in highland areas and to groups of young civil society activists in cities.
The state security apparatus appears to be targeting anyone capable of organising opposition to the dictates of the Communist party and who has the potential to emerge as a figurehead for disparate groups of government critics.
Amnesty International said that Vietnam had become “one of the most prolific jailers of peaceful activists” in Southeast Asia, an unenviable reputation in a region known for its growing intolerance of dissent.