Vietnam police have arrested four activists.
Critics say the arrests on Sunday demonstrate Vietnam's ruling Communist party's growing intolerance for any criticism.
The activists were arrested on charges of engaging in "activities aimed at overthrowing the government."
Arrested were Pham Van Troi, Nguyen Trung Ton, Truong Minh Duc and Nguyen Bac Truyen. It was not immediately clear if they have legal representation. All four have a connection to lawyer Nguyen Van Dai who was arrested in 2015 for anti-state propaganda.
A recent spate of arrests and convictions of activists has resulted in sentences as long as ten years.
Ted Osius, the U.S. ambassador in Vietnam, says he is concerned about the "deeply troubling" arrests, convictions and harsh sentences of peaceful activists.
Hanoi says there are no political prisoners in Vietnam, and only law breakers are jailed.