Vietnamese police detained at least 20 people on Sunday as they broke up a protest in Hanoi against Beijing's territorial claims in the disputed South China Sea, witnesses said.

Demonstrators were forced into waiting buses and taken to a rehabilitation centre usually used to detain sex workers and drug users, after attempting to gather in defiance of a heavy police presence, one detainee told AFP.

"There are at least 25 people here and there are arrestees elsewhere," the person -- who requested anonymity for security reasons -- said by telephone from the Loc Ha detention centre.

Another eyewitness estimated that 20 people had been detained.

Before being forcibly dispersed, the activists shouted "Down with China's aggression!" and waved Vietnamese flags and banners.

The protest is the fourth such rally in just over a month to be staged by activists in Hanoi. There were no arrests at the previous three.

Human Rights Watch said that at least four prominent bloggers and one elderly anti-corruption activist had been held for attending the latest rally, calling on the government to immediately release all those detained.

The arrests show Vietnam is "trampling on its commitments to respect civil and political rights guaranteed by international human rights treaties ratified by the government," HRW's Asia deputy director Phil Robertson said.

"Vietnam shows time and time again the yawning gap in international human rights standards between what governments say they do and what they actually do on the ground," he said.

The communist country "is rapidly racing to the bottom of the heap in Southeast Asia when it comes to violating human rights," he added.

The demonstrations come at a time of rising regional tensions over the South China Sea, which is believed to contain vast oil and gas deposits.

Hanoi and Beijing have a long-standing territorial dispute over the Spratly and Paracel Islands, which both countries claim, and frequently trade diplomatic barbs over oil exploration and fishing rights.

Relations between the pair have soured recently, with Vietnam attracting China's ire last month after it adopted a law that places the Spratlys under Hanoi's sovereignty.

China's state-backed China National Offshore Oil Corp. also said it was seeking bids for exploration of oil blocks in disputed waters -- a move slammed by Vietnam.

Vietnam, a one-party Communist state, last year allowed a number of anti-China rallies to go ahead without interference, but later clamped down, briefly detaining dozens of people.

China says it has sovereign rights to the whole South China Sea, which also has major international shipping routes. The sea is also subject to overlapping claims by Taiwan, Vietnam, Brunei, Malaysia and the Philippines.