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HANOI (Reuters) - Police in Vietnam’s capital stopped an anti-China protest within minutes on Thursday at a ceremony to commemorate a clash between the two countries in the South China Sea more than four decades ago.

Vietnam and China have a longstanding dispute over the South China Sea, nearly all of which is claimed by China. Four other countries have claims in the sea, through which an estimated $5 trillion in trade passes each year.

The protest in Hanoi started after a peaceful commemoration for soldiers of what was then South Vietnam who were killed in 1974, when China seized the Paracel islands, which it still holds.

Police dragged about 20 protesters on to a bus after they ignored a warning to disperse and began marching with banners and chanting “Demolish China’s Invasion” and other slogans.

The government and police made no comment and state-controlled media did not report the protest.

Tension between the two communist countries peaked most recently in 2014, when China moved an oil rig into disputed waters and protests broke out across Vietnam.

Relations have since improved, although a quiet military buildup continues in the region.

China and Vietnam last week pledged to manage their differences and safeguard peace in the South China Sea during a visit to Beijing by Vietnamese Communist Party chief Nguyen Phu Trong.

The potential for the busy waterway to become a global flashpoint was highlighted last week when the nominee for U.S. Secretary of State, Rex Tillerson, told a Senate hearing that China should be denied access to islands it has built there.