Police burst into a private musical performance in the city and attacked the participants, singling out the pro-democracy campaigner, Pham Doan Trang, the singer, Nguyen Tin, and Nguyen Dai for particularly brutal treatment.
The three were hooded and taken off to separate police stations where they said they were subjected to further mistreatment.
Doan Trang said she was later dumped at night by the road side in a far-flung suburb and again slapped and punched in the face.
She went to hospital where she was treated for cuts and bruises and checked for head injuries.
Trang is a well known blogger from Hanoi who has been harassed for years for her refusal to conform to the Communist party’s dictates.
She had been attending a performance in a coffee shop of love songs dating from the period before the communist victory over the south in 1975.
The singer, Nguyen Tin, and Nguyen Dai also said they were blindfolded and beaten by police officers during their interrogations.
“When the crackdown on Vietnam’s civil society reaches the point of beating and torturing people for listening to love songs, it is clear the situation is deteriorating to a disturbing level,” said Clare Algar, Amnesty International’s Director of Global Operations.
Political activists have been subjected to increasingly frequent assaults in recent years, but normally by plainclothes proxies working on behalf of the police.
The latest raid shows the growing boldness of the police to carry out violent attacks themselves.
The authorities appear to have been alarmed by the scale of protests in mid-June when tens of thousands of people took to the streets of major cities to protest against new cyber controls and the creation of economic zones they fear will give favourable treatment to Chinese companies.
The biggest demonstrations took place in Ho Chi Minh City, which the authorities fear could turn into a hub of resistance as the Communist party struggles to shore up its legitimacy.