At least 20 anti-China protesters were detained in the Vietnamese capital on Sunday, in the latest sign of the communist nation’s crackdown on dissent.

Security forces moved in to disperse about 100 demonstrators who gathered in Hanoi to protest over the ramming of a Vietnamese trawler by Chinese navy vessels in contested waters. Police blocked off rallying points and quickly put protesters on to waiting buses, according to Reuters.

Two Vietnamese journalists working for foreign media were also detained but were released after a few hours, AFP reports.

The recent flare up comes amid simmering tensions between the communist neighbors over the Spratly and Paracel Islands, which both countries claim.

Vietnamese Prime Minister Nguyen Tan Dung, speaking at a security conference in Singapore on Friday, slammed “groundless” territorial claims on Asian waters and called for self-restraint among nations, in an apparent swipe at China.

But diplomats and experts say the communist regime is eager to curtail all protests, even those against rival China, fearing they could trigger anti-government movements.