Social media websites such as Facebook are stirring populist anger in Vietnam about its powerful neighbour China, adding to tensions that have triggered skirmishes in disputed energy-rich waters.

Vietnam's authoritarian government acquiesced and allowed the largest public demonstration in four years on Sunday against Vietnam's traditional rival, with 300 people gathering outside Beijing's embassy in Hanoi.

A similar demonstration occurred in Ho Chi Minh City to oppose an alleged intrusion by Chinese ships into Vietnamese waters, and one protester said some activists talked of holding another rally this weekend.

"I think Facebook is the major communication channel," said the demonstrator, who did not want to be named.

People want a more "hardline" government approach to China, added the person, who joined Sunday's rally. "At least, they should be more vocal."

Facebook posts referred to the flow of Chinese products to Vietnam as "a cancer" and said China had behaved like "pirates" in the South China Sea.

Vietnam usually takes a tough line against public protests and tightly controls the Internet, so the fact it tolerated the rallies against China has been seen as an indication of the authorities' displeasure with Beijing.

But officials also fear further public action could backfire and get out of hand, analysts say.

"Up to a point I think it serves Vietnam's purpose but on the other hand it invites a Chinese reaction," said Carl Thayer, a Vietnam analyst based in Australia.

Tensions between Beijing and Hanoi are at their highest in years over long-standing sovereignty disputes in the potentially oil-rich Paracel and Spratly archipelagos and surrounding waters.

The situation escalated in late May after Vietnam accused three Chinese marine surveillance vessels of severing the exploration cables of an oil survey ship in its 200 nautical mile exclusive economic zone.

China said Vietnamese vessels were operating "illegally" and urged the country to "refrain from creating trouble".

The two countries traded accusations again this week over a confrontation between a Chinese fishing boat and a Vietnamese survey ship.

US Defense Secretary Robert Gates warned last weekend that clashes may erupt in the South China Sea unless nations with conflicting territorial claims adopt a mechanism to settle disputes peacefully.

Thayer said further anti-China protests might spark Chinese protests against Vietnam and aggravate relations with Vietnam's fellow communist neighbour.

In December 2007, 250-300 protesters rallied outside the Chinese embassy in Vietnam on two consecutive weekends until Beijing said bilateral ties were at risk and Hanoi asked its people to stop protesting.

Ian Storey, a regional security analyst at the Institute of Southeast Asian Studies (ISEAS) in Singapore, said the latest public protest fit a strategy by Vietnam to show its displeasure with China.

But the regime does not want regular demonstrations "because the students could start protesting about other things," Storey said.

"We think that maybe they are afraid of some protests like North Africa or the Middle East," said the protester.

Vietnamese bitterly recall 1,000 years of Chinese occupation and, more recently, a 1979 border war.

While many routinely express dislike for the Chinese, the maritime dispute has prompted a renewed outburst of patriotic sentiment in state-controlled media, independent blogs and on Facebook.

The government allegedly began blocking the social networking site in November 2009 but many Vietnamese netizens still find ways to access it, although some users reported difficulties after the rally.

Analysts differ on whether anti-China sentiment is more virulent now or in 2007 but there is no shortage of emotional comment on the Internet.

"Oh, my dear compatriots, do we continue to submit to this humiliation?" said a post on the popular Ba Sam blog.

Analysts say the government has to balance its relationship with China by not overly-offending its giant communist neighbour while avoiding the appearance of weakness before its own people.

"But at the same time no Vietnamese leader would like to be seen as guilty of appeasement," said David Koh, a Vietnam analyst from ISEAS.