The US government has said it was "deeply concerned" by the arrest in Vietnam of activist lawyer Le Cong Dinh and has called for his release.
The Paris-based group, Reporters Without Borders, also called for his immediate release, noting his work in defending bloggers and activists.
Mr Dinh was arrested on 13 June on charges of "distributing propaganda against the state".
Officials said he was arrested over his defence of pro-democracy activists.
They also said he was arrested over his use of the internet to express his views.
"Vietnam's arrest of Mr Dinh contradicts the government's own commitment to internationally-accepted standards of human rights and to the rule of law"
The police said they will charge Mr Dinh with article 88 of Vietnam's criminal code for distributing anti-government materials.
Mr Dinh, 41, one of Vietnam's most respected lawyers, has defended a number of pro-democracy activists in the past.
"Mr Dinh is a well-respected member of the Vietnamese and international legal communities, and a former Fulbright scholar," US State Department spokesman Ian Kelly said.
"No individual should be arrested for expressing the right to free speech, and no lawyer should be punished because of the individuals they choose to counsel," he said.
Mr Kelly said the arrest contradicts the Vietnamese government's "own commitment to internationally-accepted standards of human rights and to the rule of law".
He urged the Vietnamese government to release Mr Dinh immediately and unconditionally, as well as all other prisoners in detention for peacefully expressing their views.
Reporters Without Borders noted that Mr Dinh had defended several bloggers and free-speech activists, and faces a long prison sentence for his articles and commentaries in the Vietnamese press and online.
The Paris-based press freedom group reported several sources saying that his arrest could be linked to a complaint filed by several Vietnamese lawyers against the government about the granting of a bauxite mine concession to a Chinese company, a case which has caused an outcry in Vietnam.
"We fear that this arrest is aimed at punishing a respected man who promotes the cause of the rule of law in Vietnam," Reporters Without Borders said.
Le Cong Dinh has defended lawyers Nguyen Van Dai and Le Thi Cong Nhan, and the renowned blogger Dieu Cay.