Police broke up an attempt to commemorate the anniversary of a bloody clash in which 64 Vietnamese sailors were killed by Chinese forces 29 years ago.
At least two people were beaten by police after being arrested, others were prevented from leaving their homes to attend the ceremony.
Witnesses said that some 40 people did manage to gather at the memorial to Hanoi’s founding father, Ly Thai To. They offered flowers and lighted incense to honour the dead servicemen.
Large numbers of police, both in uniform and in plainclothes, were already at the scene.
They used loudhailers to demand that the mourners leave the area, and then moved in to disperse them and make arrests.
Two young participants, Nguyen Viet Dung and Do Thanh Van, said they were beaten by police after being arrested. Photographs posted on social media showed them with bloodied faces while they received medical treatment.
The veteran activist and pro-democracy campaigner, Nguyen Quang-A, said he was arrested shortly after arriving at the monument, placed in a car and driven away from the area.
In an angry post on Facebook after his release, he accused the police of kidnap, saying they failed to give any legitimate reason for his detention.
A small group managed to reconvene at a muddy bank of the Red River and launched wreaths onto the water.
The 64 Vietnamese sailors were killed on March 14, 1988, as they attempted to defend the partially submerged Johnson South reef from Chinese occupation.
Shortly afterwards the Chinese completed their occupation of the reef and five other features in the Spratly chain, outposts which have recently been transformed into potentially formidable military bases by the Chinese.
However, the Communist authorities in Hanoi have become increasingly sensitive to expressions of anti-China sentiment. They are attempting to ease tension with Beijing after a prolonged period of confrontation over the Spratly Islands and other disputed territories in the South China Sea.
The government appears concerned that civil society activists will try to undermine its legitimacy by highlighting its equivocal relationship with Beijing.
Last week police disrupted another commemoration held in the southern coastal city of Vung Tau. They seized wreaths which the participants had launched into the sea, and temporarily detained three participants.