More than 100 people gathered in Hanoi on Wednesday to commemorate the anniversary of the start of Vietnam's brief but bloody border war with China.

Thirty seven years ago, 600,000 Chinese troops invaded northern Vietnam to "teach Vietnam a lesson" for ousting the Beijing-backed Khmer Rouge in Cambodia. China withdrew its troops after a month.

The residents lit incense Wednesday and laid flowers at the statue of King Ly Thai To, a Vietnamese hero, in a ceremony that lasted an hour. They chanted "down with the aggressors," and "Hoang Sa, Truong Sa," the Vietnamese terms for the Paracel and Spratly islands in the South China Sea.

The Paracels are claimed by Vietnam, China and Taiwan, while the Spratlys are claimed in all or parts by those three along with the Philippines, Malaysia and Brunei.

The commemoration took place even though large gatherings without government approval are often stopped by the police.

There have been no official government commemorations of the war.

"The Vietnamese Communist Party and government ... appear to forget about the border war with China and the day when China occupied Hoang Sa and Truong Sa," said one participant, Nguyen Huu Hop.

Hop said he volunteered to go to the border fight the Chinese, but that his unit was instead stationed in the capital Hanoi. He said many young men in his neighborhood were killed in the war.

After China withdrew its troops, periodical fighting continued along the border for another 10 years before the communist neighbors normalized diplomatic relations in 1991.

China is now Vietnam's largest trading partner, but relations have plunged following China' parking of an oil rig near the Paracels in May 2014.

The countries have since then tried to mend ties by exchanging high-level visits, but tensions remain over the islands.