Protests are rare in Vietnam
China, Vietnam, the Philippines, Malaysia, Brunei and Taiwan all claim territories in the South China Sea.
It includes important shipping routes and may contain oil and gas deposits.
China's claim is by far the largest.
'Invasion' Tensions between China and Vietnam escalated after an incident on 26 May in which Hanoi accused a Chinese patrol of cutting the cables of a Vietnamese ship conducting seismic research about 120km (80 miles) off Vietnam's south-central coast.
The BBC's Nga Pham reports from Singapore that Vietnam and China have had a long and turbulent history and the Vietnamese are traditionally suspicious of the Chinese.
But attempts to protest publicly in the past were quashed by the authorities as Hanoi tried not to offend its big neighbour, our reporter says.
Public protests happen rarely in Vietnam, but calls for demonstrations have spread on the internet and via mobile phones in the past few days.
On Sunday, the protesters in Hanoi shouted slogans including "The Paracels and Spratlys belong to Vietnam", a reference to two groups of islands claimed by both countries.
They also carried signs that read: "Stop Chinese invasion of Vietnam's islands."
They marched to the Chinese embassy, where Vietnamese police watched them for a time before leading them away.
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