Vietnamese activists have marked the 35th anniversary of a bloody border war with China, chanting slogans, singing patriotic songs and laying flowers at a temple in central Hanoi.

The two communist countries are locked in long-standing territorial disputes over the Paracel and Spratly islands in the South China Sea, and often trade diplomatic barbs over oil exploration and fishing rights in the contested waters.

Beijing’s increasingly assertive stance in the South China Sea has triggered public anger and rare protests in authoritarian Vietnam where the demonstrations are sometimes allowed to go ahead and on other occasions forcefully broken up.

China invaded Vietnam’s northernmost provinces in February 1979, angered by Vietnam’s ousing of the Beijing-backed Khmer Rouge regime in Cambodia.

The short but bloody conflict claimed tens of thousands of lives on both sides and ended with Chinese forces withdrawing and both Hanoi and Beijing claiming victory.

Vietnamese troops remained in Cambodia until 1989.

Although Vietnam fetes its military victories over the French and American armies, it has not arranged any official events to mark the China border war – much to the chagrin of veterans and activists.

“Vietnamese leaders may have received pressure from China, so they don’t want to talk about that war. They seem to want to deny the past,” Nguyen Trong Vinh, a former Vietnamese ambassador to China, told AFP.

China has long been one of Vietnam’s largest trading partners, state media has said, with bilateral trade at more than $US40 billion ($A45 billion) in 2012.

On Sunday, around 100 activists tried to lay flowers at a statue of Ly Thai To – the founder of Hanoi and a nationalist figurehead – in the centre of the capital.

But dozens of people had been at the monument since early morning, playing loud music and dancing, which prevented the protesters from holding their planned ceremony – in what activists said was a counter protest.

“It was deliberate… they (authorities) hired many people,” economist Nguyen Quang told AFP at the protest.

Protesters, wearing red headbands and carrying white roses with black ribbons saying “the people will never forget”, then walked around the central Hoan Kiem lake.

They laid their flowers and made brief speeches at the Ngoc Son Temple – a popular tourist destination – before peacefully dispersing.

Plain clothed and uniformed police closely monitored the event but did not make any arrests.