Vietnam arrested a prominent human rights activist on Tuesday, accusing him of distributing anti-state propaganda.
Nguyen Lan Thang is well known in the communist country for his social activism as well as his online criticism of the government, which has become increasingly intolerant of any dissent.
The 46-year-old was arrested at his home in Hanoi "for making, hoarding, disseminating and spreading propaganda against the state of the Socialist Republic of Vietnam under article 117 of the 2015 Penal Code", the capital's police said in an official statement.
Officers did not provide any other details, stating that the case was currently under investigation.
Vietnam is notorious for its harsh treatment of those holding viewpoints diverging from the official pro-government line.
Thang has endured short spells in detention, as well as harassment from officers, in the past.
"I'm a focal point for police," Thang previously told the Committee for Protecting Journalists.
Human Rights Watch on Tuesday called for his "immediate and unconditional release".
"Vietnam's outrageous and unacceptable crackdown on freedom of expression has just snared another victim who will invariably face a kangaroo court trial and years in prison for speaking his mind," said deputy Asia director Phil Robertson.
He added "Thang's peaceful advocacy for democratic reforms and justice should be respected and listened to rather than face this kind of unjustified repression."
Social media reports claimed that authorities had also confiscated digital devices, as well as human rights-related books, from Thang's home during the arrest.
His family could not be reached for comment.