Protests - which are not common in authoritarian Vietnam - have taken place in the capital Hanoi for five weekends in a row over the maritime dispute.

Plainclothes and uniformed security agents poured into the area around the Chinese embassy and sealed off surrounding roads, but about 40 protesters penetrated the cordon to gather some distance from the embassy. They marched towards central Hanoi, trailed by riot police and other security personnel.

More people joined the rally along the way, shouting that two South China Sea archipelagos, the Spratlys and Paracels, belong to Vietnam.

The two countries have longstanding sovereignty disputes over the potentially oil-rich island groups, which also straddle commercial shipping lanes vital to global trade.

Tensions flared and protests began after Hanoi in late May accused Chinese marine surveillance vessels of cutting the exploration cables of an oil survey ship inside its exclusive economic zone.

In a second incident, Hanoi alleged that a Chinese fishing boat rammed the cables of another ship in the 200-nautical-mile zone. Vietnam then held a live-fire naval drill off its central coast.

"China must respect Vietnam's exclusive economic zone," said a sign held aloft by the demonstrators.

"Anti-China", another said simply, in English.

Analysts have said that Vietnamese authorities permitted the anti-China rallies, which in the past have drawn up to 300 people, because they served the government's purpose of expressing displeasure with Beijing over the dispute.

Two protests took place in southern Ho Chi Minh City, but there have been none there since June 11. A person involved in the rallies told AFP that security forces had "intimidated" people there.

In Hanoi, some activists feel their demonstrations were "in vain" after China and Vietnam held talks on June 25 in Beijing, said the source, who spoke on condition of anonymity.

State media from both countries said the two sides agreed at the talks to resolve their maritime territorial disputes peacefully "through negotiations and friendly consultations".

The official Vietnam News said that Beijing and Hanoi "also laid stress on the need to steer public opinion in the correct direction".

That means Vietnam must rein in the demonstrators, while China should control its media, whose comments on the maritime issue have upset Vietnam, said Carl Thayer, a longtime Vietnam analyst based in Australia.

But officials could be divided "about how to carry out the intent of the joint press release", said Thayer, expressing surprise that a fresh protest had occurred after the Beijing-Hanoi talks.

"Perhaps it is too early, and Hanoi and Beijing are watching what steps the other takes," he said.

Thayer noted that the protests had already appeared to dwindle in size, and said that if authorities follow a precedent set during anti-China rallies four years ago, "security officials will visit schools and universities and warn students that they risk expulsion" for protesting.

China is Vietnam's largest trading partner, but Vietnamese bitterly recall 1,000 years of Chinese occupation and, more recently, a 1979 border war.

The Philippines, Malaysia, Brunei and Taiwan have also staked claims to the Spratlys.

By Ian Timberlake - Agence France Presse - July 3, 2011