About 200 protesters brought parts of central Hanoi to a brief halt on Sunday in the third rally this month against Beijing's perceived territorial assertiveness in the South China Sea.

Demonstrators shouting "Down with China's aggression!" brought traffic around Hoan Kiem Lake in the centre of the Vietnamese capital to a standstill as they marched towards the Chinese embassy.

They were prevented from getting close to the mission building by security forces -- who broke up similar rallies last year -- but no arrests were made, according to witnesses.

The protests come at a time of rising regional tensions over the South China Sea, which is believed to contain vast oil and gas deposits and is the subject of a web of competing claims between Beijing and its neighbours.

On Friday, Southeast Asian nations vowed to work towards a "code of conduct" in the disputed waters, a week after divisions over the territorial issues marred a regional ministerial meeting in Phnom Penh.

Vietnam and the Philippines have recently accused Beijing of increasingly aggressive behaviour in the South China Sea.

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.

Protests are rare in authoritarian Vietnam.

Anti-China rallies last year were allowed to go ahead without interference initially, but authorities later clamped down, briefly detaining dozens of people after talks between Hanoi and Beijing.

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.