Around a dozen activists briefly waved banners and shouted "Chinese oil rig get out of Vietnam!" and "silence is cowardice" - a dig at Hanoi's handling of the dispute - before police forced them to disperse.
Around seven activists were detained by police, according to reports posted on activist blogs. Police did not confirm the detentions.
"It is not good for people to gather here... We need to avoid causing difficulties for the state. The state can protect Vietnam's sovereignty," Do Anh Vinh, 22, a communist youth volunteer helping security forces said.
The two sides have traded accusations in an increasingly heated territorial dispute, with each side claiming the other has engaged in aggressive behaviour against its ships, including by ramming them.
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China's top foreign policy official, State Councillor Yang Jiechi met Vietnam's Foreign Minister and Prime Minister Nguyen Tan Dung in Hanoi yesterday but the talks ended in deadlock.
Vietnam's communist leaders have struggled to balance strong domestic opposition to China's unilateral moves in the South China Sea with their traditionally friendly ties with a fellow communist country.
Hanoi has since moved to muzzle public protest as it seeks to reassure nervous foreign investors that the country is a safe place to do business.