Vietnam broke up an anti-China rallies in its two main cities and arrested around 20 people Sunday, activists told AFP, in the latest protests against Beijing as tensions flare over the South China Sea.

Around 200 protesters, some waving banners and chanting "Down with China's aggression!", were intercepted by security forces as they tried to approach the Chinese embassy in the capital.

Activists at the scene told AFP that the 20 demonstrators were rounded up into a bus after the half-hour rally, the fifth such display of public discontent in Hanoi this year against Beijing's perceived aggression in the sea.

The protestors were taken to Loc Ha detention centre on the outskirts of Hanoi, one activist told AFP on condition of anonymity. "I'm detained. And they've jammed my mobile phone," the activist said via text message. They could not be reached for further questions.

Security forces also broke up a similar anti-China protest also attended by hundreds of people in the southern economic hub of Ho Chi Minh City, eyewitnesses told AFP, after Hanoi and Beijing traded barbs over the neighbours' conflicting territorial claims.

The South China Sea is strategically significant, home to some of the world's most important shipping lanes and believed to be rich in resources.

Vietnam, which has begun exploring for oil in what it claims as its territorial waters, last week issued a rebuke to Beijing over claims that Chinese fishing boats had sabotaged a boat operated by state-run energy giant PetroVietnam.

China responded by denying the allegations and demanding that Vietnam end oil exploration and stop its navy harassing Chinese boats.

Beijing's increasingly assertive stance on the South China Sea has stoked public anger in Vietnam and given way to rare protest in the authoritarian country.

Rallies, which began in 2011, were initially tolerated by Hanoi, but authorities clamped down on later gatherings, briefly detaining dozens of people after talks between Hanoi and Beijing.

The first demonstrations this year were allowed to go ahead, but in August Vietnamese police detained some 25 people at an anti-China rally who were later taken to a rehabilitation centre usually used to detain sex workers and drug users.

Vietnam and China have a long-standing territorial dispute over the Spratly and Paracel Islands, which both countries claim.

Beijing says it has sovereign rights to virtually all of the South China Sea, which is also claimed in whole or part by Taiwan, Vietnam, Brunei, Malaysia, and the Philippines.