Scores of activists were detained in Vietnam's capital Sunday as they gathered to protest against China after it rejected a recent international ruling that dismissed its claims to much of the South China Sea.

Anti-Chinese sentiment runs deep in communist Vietnam but the country's authoritarian rulers move swiftly to tamp down expressions of public anger, fearful that allowing such protests might embolden criticism of their rule.

Activists had used social media to call for protests in Hanoi on Sunday in the wake of this week's ruling by the Permanent Court of Arbitration (PCA) in The Hague which found there was no legal basis for Beijing's claims to vast swathes of the South China Sea.

The case was brought by the Philippines but the ruling has been a boon for other regional countries like Vietnam who also have competing claims to the strategic sea.

Authorities in the capital Hanoi were ready for protests on Sunday.

Plainclothes security forces were out in force, blanketing much of the city centre and keeping a close eye on any crowds that might be gathering, an AFP reporter on the scene said.

Throughout the morning around 30 activists were swiftly bundled onto waiting buses and cars by security forces after they gathered to hold a protest near the city's famous Hoan Kiem lake, a common spot for demonstrations.

Some chanted "Down with China invasion!" as they were led away to detention.

Later in the day a group of nine activists held a brief rally outside the Philippines embassy. One held aloft a banner that read: "Thank you Philippines. You have a brave government."

The group dispersed on motorbike before security forces arrived.

Activists posted pictures on social media of similar flashmob rallies in southern Ho Chi Minh City with protesters riding around the city on motorbikes holding banners criticising China.

Beijing lays claim to virtually all of the South China Sea, putting it at odds with regional neighbours the Philippines, Vietnam, Malaysia, Brunei and Taiwan, which also have partial claims.

China boycotted the PCA hearings, saying the court had no jurisdiction, and has reacted furiously, vowing to ignore the ruling and arguing it misinterprets international law.

Vietnam and China frequently trade diplomatic barbs over the disputed Paracel island chain and waters in the South China Sea.

China has encouraged patriotic citizens to visit the contested Paracels, which are known as Xisha in Chinese.

Such acts have deepened already simmering anti-Chinese sentiment in Vietnam while domestic critics accuse Hanoi of being too meek towards its giant northern neighbour.

At least three Chinese nationals were killed in 2014 when rioting broke out in Vietnam after Beijing sent an oil rig into contested waters.