Vietnam yesterday detained about 30 anti-China protesters in Hanoi, amid growing tensions over rival territorial claims in the South China Sea.
Security forces moved in to disperse about 100 demonstrators who gathered in the city centre for an hour-long rally, witnesses said.
Those detained were taken by bus to a "rehabilitation" centre on the outskirts of the capital.
Vietnam and China have long-standing territorial disputes over the Spratly and Paracel Islands, which both countries claim, and often trade diplomatic barbs over oil exploration and fishing rights in the contested waters.
In March, Hanoi accused a Chinese vessel of firing on one of its fishing boats.
"This is not China's sea. That's why we're protesting against this harsh aggression," protester Tam Trong Khang, 64, said.
Last month Beijing sent one of its largest recorded fishing fleets to the disputed waters.
More than a dozen anti-China demonstrations have been held in Vietnam since 2011. After initially tolerating the rallies, the Vietnamese authorities have since banned them.
Vietnamese Prime Minister Nguyen Tan Dung, speaking at a security conference in Singapore, on Friday slammed "groundless" territorial claims in Asian waters and called for self-restraint among nations involved in disputes.