In a rare show of defiance against a police crackdown on dissent, hundreds of people took to the streets in the north-central province of Ha Tinh to demand the release of a local activist.
Some reports said that more than two thousand people took part in the march in the coastal region of Cua Sot to show their support for civil society campaigner, Tran Thi Xuan, who was arrested last week.
Ms Xuan is being investigated on a charge of attempting to overthrow the people’s administration under Article 79 of the penal code.
She is thought to have been targeted because of her work with local Catholic priests, who have been organising support for local people who suffered from the aftermath of an environmental disaster last year.
The police have not been specific about the charges against Xuan, but reports in official media alleged she had liaised with foreign based organisations that tried to assist local fishermen.
She is also accused of belonging to the Brotherhood for Democracy, a group that presses for more pluralistic government, and whose leading members have already been arrested.
The government is acutely sensitive to the actions of some foreign based groups, which it accuses of attempting to stir up disorder.
Large crowds also attended a prayer vigil for the activist at Cua Sot Church on Wednesday night, the day after her arrest.
Some held up banners demanding her release.
Catholic priests have helped organise a protest movement on the coastline to help local people demand compensation for losses suffered in the disaster last year.
Toxic chemicals leaking from a Taiwanese owned steel plant on the coast had a devastating impact on the fishing industry along the central coast.